American and European intelligence analysts now believe that President Bashar al-Assad’s troops have used chemical weapons against rebel forces in the civil war there, an assessment that will put added pressure on a deeply divided Obama administration to develop a response to a provocation that the president himself has declared a “red line.”

According to an internal memorandum circulating inside the government on Thursday, the “intelligence community assesses that the Assad regime has used chemical weapons on a small scale against the opposition multiple times in the last year.” President Obama said in April that the United States had physiological evidence that the nerve gas sarin had been used in Syria, but lacked proof of who used it and under what circumstances. He now believes that the proof is definitive, according to American officials.

But a flurry of high-level meetings in Washington this week only underscored the splits within the Obama administration about what actions to take to quell the fighting, which has claimed more than 90,000 people. The meetings were hastily arranged after Mr. Assad’s troops — joined by fighters from the militant group Hezbollah — claimed the strategic city of Qusayr and raised fears in Washington that large parts of the rebellion could be on the verge of collapse.

Senior State Department officials have been pushing for an aggressive military response, including airstrikes to hit the primary landing strips in Syria that the government uses to launch the chemical weapons attacks, ferry troops around the country, and receive shipments of matériel from Iran. But White House officials remain wary, and one American official said that a meeting on Wednesday of the president’s senior advisers yielded no firm decisions about how to proceed.

It is unclear precisely how the Obama administration made its final determination about the chemical weapons use in Syria. According to the internal memorandum, intelligence agencies have “high confidence” in their assessment, and estimate that between 100 and 150 people have died to date from chemical weapons attacks. The memorandum goes on to say that the conclusion is based on a variety of intelligence.

“Our intelligence community has high confidence in that assessment given multiple, independent streams of information,” the memorandum said.

After dragging 46 bodies from the streets near his hometown on the Syrian coast, Omar lost count. For four days, he said, he could not eat, remembering the burned body of a baby just a few months old; a fetus ripped from a woman’s belly; a friend lying dead, his dog still standing guard.

Omar survived what residents, antigovernment activists and human rights monitors are calling one of the darkest recent episodes in the Syrian war, a massacre in government-held Tartus Province that has inflamed sectarian divisions, revealed new depths of depravity and made the prospect of stitching the country back together appear increasingly difficult.

That mass killing this month was one in a series of recent sectarian-tinged attacks that Syrians on both sides have seized on to demonize each other. Government and rebel fighters have filmed themselves committing atrocities for the world to see.

Footage routinely shows pro-government fighters beating, killing and mutilating Sunni rebel detainees, forcing them to refer to President Bashar al-Assad as God. One rebel commander recently filmed himself cutting out an organ of a dead pro-government fighter, biting it and promising the same fate to Alawites, members of Mr. Assad’s Shiite Muslim sect.

That lurid violence has fueled pessimism about international efforts to end the fighting. As the United States and Russia work to organize peace talks next month between Mr. Assad and his opponents, the ever more extreme carnage makes reconciliation seem more remote.

Nadim Houry, the director of Human Rights Watch in Beirut, said he sensed “a complete disconnect between diplomacy and events on the ground.”

“The conflict is getting more visceral,” he said. Without concrete confidence-building measures, he said, and with more people “seeing it as an existential struggle, it’s hard to imagine what the negotiations would look like.”

Israel’s reported airstrikes in Syria — and the threat of a retaliatory strike by the Syrian government — are likely to accelerate the decision-making of the Obama administration, which was already moving toward a sharp escalation of U.S. involvement in the two-year-old crisis.

Senior officials said the deployment of U.S. troops to Syria remains unlikely, but they have indicated that a decision will come within weeks on options ranging from the supply of weapons to the Syrian rebels to the use of U.S. aircraft and missiles to ground President Bashar al-Assad’s air power by destroying planes, runways and missile sites inside Syria.

Neither Israeli nor U.S. officials confirmed an attack Sunday morning that reportedly hit a weapons shipment in Syria — including sophisticated missiles and air defense equipment — about to be transferred to Lebanon-based Hezbollah.

But President Obama, in an interview broadcast just hours later Sunday, said Israel is justified in preventing the provision of weapons to Hezbollah.

“We coordinate very closely with the Israelis, recognizing that . . . they are very close to Syria, they’re very close to Lebanon,” Obama said in the interview, recorded Saturday with the Spanish-language Telemundo, after an earlier Israeli attack reported late Friday.

Throughout the Syrian crisis, the administration has repeatedly voiced the belief that Syria is already awash in weapons and that sending more will not tip the balance in favor of the rebels.

Now, in part because of growing confidence in the rebel Free Syrian Army, “the national security team and the diplomatic team around the president” favor increased involvement, and their views are gaining momentum despite the caution expressed by Obama’s political advisers, according to a senior Western official whose government has closely coordinated its Syria policy with Washington and who spoke before the reported Israeli strikes. The official discussed sensitive diplomatic assessments on the condition of anonymity.

The Syrian government publicly condemned Israel for a powerful air assault on military targets near Damascus early Sunday, saying it “opened the door to all possibilities,” as fear spread throughout the region that the country’s civil war could expand beyond its borders.

The attack, which sent brightly lighted columns of smoke and ash high into the night sky above the Syrian capital, struck several critical military facilities in some of the country’s most tightly secured and strategic areas, killing dozens of elite troops stationed near the presidential palace, a high-ranking Syrian military official said in an interview.

Israel refused to confirm the attacks, the second in three days, and Israeli analysts said it was unlikely that Israel was seeking to intervene in the Syrian conflict. They said the attacks in all likelihood expanded and continued Israel’s campaign to prevent the Syrian government from transferring weapons to Hezbollah, the Shiite militia and political party in neighboring Lebanon that is one of Israel’s most dangerous foes.

Rebels, opposition activists and residents said the strikes hit bases of the elite Republican Guard and storehouses of long-range missiles, in addition to a military research center that American officials have called the country’s main chemical weapons facility.

An American official said a more limited strike early Friday at Damascus International Airport was also meant to destroy weapons being sent from Iran to Hezbollah.

Concerns flared about whether Hezbollah might attack Israel in retaliation, possibly drawing Lebanon into the conflict. Israel deployed two of its Iron Dome missile-defense batteries in its northern cities. Iran’s IRNA news agency said Israel could expect a “crushing” retaliation from Syria or “the resistance,” meaning Hezbollah.

Analysts said Syria, weakened by the conflict, and Hezbollah, overstretched as it commits more forces to support the Syrian government, were unlikely to act, but they cautioned that a miscalculation by either side that set off an escalation could not be ruled out. And President Bashar al-Assad could choose to mount covert attacks on Israeli targets abroad, rather than engage its military directly.

One senior Israeli official, who spoke on the condition of anonymity, said he did not think that Israel was entering a war with Syria and suggested that Syria was unlikely to respond. Mr. Assad “has his own problems,” the official noted. “He doesn’t need Israel in the mess.”

A powerful explosion ripped through Marjeh Square in the center of the Syrian capital, Damascus, on Tuesday, killing 13 people and wounding over 70, according to state television, which showed video of what it said was the aftermath.

The bombing came a day after an explosion targeted the convoy of the Syrian prime minister, Wael Nader al-Halqi, in the affluent Mezze district of Damascus. State news media said that Mr. Halqi survived the attack, which killed five people, including a bodyguard.

The latest blast came when a booby-trapped car exploded near the back door of a building that used to house the Ministry of Interior, according to the Syrian Observatory for Human Rights, an organization, based in Britain, with a network of local antigovernment activists in Syria. The group said that the death toll could rise, as some of the wounded were in critical condition, and at least five members of the security forces were killed. State television said the dead were all civilians.

In footage on state television, fire trucks and ambulances could be heard in the background as a camera panned over debris, bloodstains on the ground and dented cars with broken windows. A thick spiraling cloud of back smoke engulfed the area as passers-by spoke on their cellphones and looked around in disbelief.

It was not clear if any individual was targeted, and no group immediately claimed responsibility. The government blamed its armed opponents, while the opposition Local Coordinating Committees blamed the government, as has often happened in a war in which information is a weapon and each side seeks to demonize the other.

An array of disparate groups are seeking to topple the Syrian president, Bashar al-Assad, including the blacklisted al-Nusra Front which recently pledged allegiance to Al Qaeda and has claimed responsibility for similar attacks in the past. Other rebel groups say they reject such tactics.

… As the violent civil war in Syria enters its third year and Mr. Assad’s opponents try to inch closer to Damascus, the city has witnessed increasingly frequent explosions, including powerful bombings that have targeted government officials, others whose targets appear random and occasional rebel mortars that sail into the center. Far more devastating has been the relentless bombardment of rebel-held suburbs — and other areas around the country — by security forces using artillery and airstrikes. More than 70,000 people have died in the conflict.

The Syrian government and rebels accused each other Tuesday of firing a chemical weapon near the city of Aleppo, killing at least two dozen people in an attack that, if confirmed, would mark the first use of chemical arms and a major escalation in Syria’s two-year-old conflict.

Neither claim was independently confirmed Tuesday, nor was it clear what sort of weapon had been used in the attack. The United States, which has warned President Bashar al-Assad that a chemical attack would be a “red line” that would trigger a U.S. military response, expressed deep skepticism about the reports.

Syrian Information Minister Omran al-Zoubi said rebels fired “a rocket containing poison gases” at the town of Khan al-Assal, southwest of Aleppo, from a part of the city they control. The state-run Syrian Arab News Agency (SANA) said 25 people were killed and 86 injured in the attack. Rebels, in turn, accused forces loyal to Assad of firing a Scud missile containing deadly chemicals.

The Britain-based Syrian Observatory for Human Rights said 26 people died, including 16 soldiers. But the observatory’s director, who uses the pseudonym Rami Abdulrahman, said he could only “confirm that there was a rocket attack but not that any chemicals were used.”

In Washington, White House spokesman Jay Carney said the allegations were being studied. State Department spokeswoman Victoria Nuland characterized the Syrian claim as an attempt to discredit the opposition and said the United States has no evidence that rebels have chemical weapons capabilities.

The United States is significantly stepping up its support for the Syrian opposition, senior administration officials said on Wednesday, helping to train rebels at a base in the region and for the first time offering armed groups nonlethal assistance and equipment that could help their military campaign.

The training mission, already under way, represents the deepest American involvement yet in the Syrian conflict, though the size and scope of the mission is not clear, nor is its host country. The offer of nonlethal assistance is expected to come from Secretary of State John Kerry at a meeting on Thursday in Rome with opposition leaders. Mr. Kerry is also expected to raise the prospect of direct financial aid, though officials cautioned that the White House still had to sign off on all the elements.

Before arriving in Rome on Wednesday, Mr. Kerry declared in Paris that the Syrian opposition needed additional assistance and indicated that the United States and its partners planned to provide some.

Under a broad definition of “nonlethal,” assistance to the opposition could include items like vehicles, communications equipment and night vision gear. The Obama administration has said it will not — at least for now — provide arms to the opposition.

One major goal of the administration is to help the opposition build up its credibility within Syria by providing traditional government services to the civilian population. Since the conflict erupted two years ago, the United States has sent $365 million in humanitarian aid to Syrians. American officials have been increasingly worried that extremist members of the resistance against the government of President Bashar al-Assad, notably the Al Nusra Front, which the United States has asserted is affiliated with Al Qaeda, will take control of portions of Syria and cement its authority by providing public services, much as Hezbollah has done in Lebanon.

“Some folks on the ground that we don’t support and whose interests do not align with ours are delivering some of that help,” Mr. Kerry said.