In a phone call today with Turkish Prime Minister Recep Tayyip Erdogan, Israeli Prime Minister Benjamin Netanyahu apologized to Turkey over a 2010 Israeli raid of a flotilla that left nine people dead. The flotilla was attempting to break an Israeli naval blockade of Gaza, when it was intercepted by Israel.

“PM Netanyahu made it clear that the tragic Consequences of the Mavi Marmara flotilla were not intended & that Israel is sorry for loss of life,” Ofir Gendelman, the prime minister’s spokesman said on Twitter. “PM Netanyahu apologized to the Turkish ppl for any mistake that could have led to loss of life, agreed to complete the compensation agreement.”

This is a tremendous diplomatic breakthrough. As we’ve reported, back in September of 2011, Erdogran said the raid in international waters was “cause for war.” Israel had refused to apologize since the incident.

In September of 2011, a United Nations panel found Israel’s naval blockade of Gaza was legal but that the raid was “excessive and unreasonable.”

According to a senior White House official, Erdogan accepted the apology. Another senior administration official said this was a “first step” toward normalization of relations between the two countries.

The phone call between Netanyahu and Erdogan took place in a trailer at the Tel-Aviv airport where President Obama was taking off for Amman. The senior administration official said at one point Obama jumped in on the call.

Syria pulled both Turkey and Israel closer to military entanglements in its civil war on Monday, bombing a rebel-held Syrian village a few yards from the Turkish border in a deadly aerial assault and provoking Israeli tank commanders in the disputed Golan Heights into blasting mobile Syrian artillery units across their own armistice line.

The escalations, which threatened once again to draw in two of Syria’s most powerful neighbors, came hours after the fractious Syrian opposition announced a broad new unity pact that elicited praise from the big foreign powers backing their effort to topple President Bashar al-Assad.

“It is a big day for the Syrian opposition,” wrote Joshua Landis, an expert on Syrian political history and the author of the widely followed Syria Comment blog. Mr. Landis, director of the Center for Middle East Studies at the University of Oklahoma, wrote that the “Assad regime must be worried, as it has survived for 42 years thanks to Syria’s fragmentation.”

There has been speculation that Mr. Assad, feeling increasingly threatened, may deliberately seek to widen the conflict that has consumed much of his own country for the past 20 months and left roughly 40,000 people dead. Although there is no indication that Mr. Assad has decided to try to lure Israel into the fight, any Israeli involvement could rally his failing support and frustrate the efforts of his Arab adversaries.

The attack on the Turkish border, by what Syrian witnesses identified as a Syrian MIG-25 warplane, demolished at least 15 buildings and killed at least 20 people in the town of Ras al-Ain, the scene of heavy fighting for days and an impromptu crossing point for thousands of Syrian refugees clambering for safety into Turkey.

“The plane appeared in seconds, dropped a bomb and killed children,” said Nezir Alan, a doctor who witnessed the bombing. “Here is total chaos.” In a telephone interview from Ras al-Ain, he said the bombing wounded at least 70 people, 50 of them critically. Turkish television stations reported that ambulances were rushing victims into Ceylanpinar, Turkey, just across the border.

Windows of shops and houses in Ceylanpinar were shattered by the force of the bombing, and Turkish television showed people on both sides of the border running in panic, while military vehicles raced down streets as a huge cloud of smoke hung over the area.

There were no immediate reports of deaths or injuries in Ceylanpinar. But the Turkish authorities, increasingly angered by what they view as Syrian provocations, have deployed troops and artillery units along the 550-mile border with Syria and have raised the idea of installing Patriot missile batteries that could deter Syrian military aircraft.

Buoyed by support from his country’s NATO allies, Prime Minister Recep Tayyip Erdogan warned Syrian forces on Tuesday to stay clear of their troubled border or face a Turkish military response to any perceived threat, following the disputed downing of a Turkish warplane.

The Turkish leader’s bellicose tone came as ambassadors from the NATO alliance, seeking to avoid a wider conflict, held emergency talks in Brussels at Turkey’s behest. After the meeting, the NATO secretary general, Anders Fogh Rasmussen, said the alliance considered Syria’s actions in shooting down a Turkish warplane last Friday “unacceptable.”

In a unanimous statement, the NATO allies called the episode “another example of the Syrian authorities’ disregard for international norms, peace and security, and human life.” Turkey is a member of the alliance.

“I would certainly expect that such an incident won’t happen again,” Mr. Rasmussen said at a news conference at NATO headquarters in Brussels. He added that the alliance would closely follow developments and “if necessary, consult and discuss what else could be done.”

In Ankara, Mr. Erdogan said Turkey had revised its military rules of engagement toward Syria.

“Every military element that approaches the Turkish border from Syria in a manner that constitutes a security risk or danger would be considered as a threat and would be treated as a military target,” he said in a speech to lawmakers attended by Arab diplomats.

“From here, we warn the Syrian regime not to make any mistakes, not to test Turkey’s decisiveness and wisdom,” Mr. Erdogan said.

Once one of Syria’s closest allies, Turkey is hosting an armed opposition group waging an insurgency against the government of President Bashar al-Assad, providing shelter to the commander and dozens of members of the group, the Free Syrian Army, and allowing them to orchestrate attacks across the border from inside a camp guarded by the Turkish military.

The support for the insurgents comes amid a broader Turkish campaign to undermine Mr. Assad’s government. Turkey is expected to impose sanctions soon on Syria, and it has deepened its support for an umbrella political opposition group known as the Syrian National Council, which announced its formation in Istanbul. But its harboring of leaders in the Free Syrian Army, a militia composed of defectors from the Syrian armed forces, may be its most striking challenge so far to Damascus.

On Wednesday, the group, living in a heavily guarded refugee camp in Turkey, claimed responsibility for killing nine Syrian soldiers, including one uniformed officer, in an attack in restive central Syria.

Turkish officials describe their relationship with the group’s commander, Col. Riad al-As’aad, and the 60 to 70 members living in the “officers’ camp” as purely humanitarian. Turkey’s primary concern, the officials said, is for the physical safety of defectors. When asked specifically about allowing the group to organize military operations while under the protection of Turkey, a Foreign Ministry official said that their only concern was humanitarian protection and that they could not stop them from expressing their views.

“At the time all of these people escaped from Syria, we did not know who was who, it was not written on their heads ‘I am a soldier’ or ‘I am an opposition member,’ ” a Foreign Ministry spokesman said on the condition of anonymity in keeping with diplomatic protocol. “We are providing these people with temporary residence on humanitarian grounds, and that will continue.”

At the moment, the group is too small to pose any real challenge to Mr. Assad’s government. But its Turkish support underlines how combustible, and resilient, Syria’s uprising has proven. The country sits at the intersection of influences in the region — with Iran, Hezbollah in Lebanon, Saudi Arabia and Israel — and Turkey’s involvement will be closely watched by Syria’s friends and foes.