Correction: An earlier version of this article misquoted the Turkish Foreign Ministry spokesman Selcuk Unal as saying the shooting down by Syria of a Turkish plane was an “act of war.” He actually said, “It was a hostile act.” This version has been corrected.

Turkish coast guard searches for the Turkish warplane which was downed by Syria on Friday off Samandagi in Hatay province in the eastern Mediterranean, Sunday, June 24, 2012. (AP/AP)

— Turkey on Sunday summoned its NATO allies for emergency consultations on the downing by Syria of one of its warplanes, a move that potentially opens the door to international military intervention in the Syrian crisis for the first time.

Turkey said it had invoked Article 4 of the NATO charter — which allows consultations in case of a security threat — after concluding that the plane was over international waters in the Mediterranean when it was hit by a Syrian missile Friday.

Investigations into the shooting suggested that it was not an accident or a mistake, and that Syria was aware it was firing at a Turkish plane when the U.S.-made F-4 fighter was targeted without warning by at least two surface-to-air missiles, Turkish officials said. A search continued in the eastern Mediterranean for the two missing pilots.

“It was a hostile act,” Turkish Foreign Ministry spokesman Selcuk Unal said in a telephone interview. “They shot down a plane over international waters, and this is unacceptable.” Turkey sent a diplomatic note to Syria stating that under international law, Turkey “reserves the right to respond,” he added.

In Washington, Secretary of State Hillary Rodham Clinton called the downing “a brazen and unacceptable act” and said the United States was consulting with its allies and partners regarding “next steps” to be taken against Syria, at a time when a U.N. effort to address the spiraling bloodshed inside Syria through diplomacy is faltering.

Although immediate military action seems unlikely, Turkey’s summons puts the Syrian crisis on NATO’s agenda for the first time since the uprising began, and the development “is very significant,” said Salman Shaikh, director of the Brookings Doha Center in Qatar.

“The preferred option for everyone including the United States is still a political solution,” he said. “But whereas a few days ago a military option was not on the cards, now it will be discussed in a way it hasn’t been for the past year and a half. It activates NATO, which we haven’t seen before.”

NATO spokeswoman Lungescu Oana said ambassadors of the alliance’s 28 member-states will meet in Brussels on Tuesday to hear a Turkish presentation on the incident.

“Under Article 4, any ally can request consultations whenever, in the opinion of any of them, their territorial integrity, political independence or security is threatened,” she said in a statement.

Turkey’s request for a NATO meeting came after two days of deliberations between Prime Minister Recep Tayyip Erdogan, his ministers and top Turkish military officials, who gave little indication as to how Turkey planned to respond to the most serious cross-border incident since the Syrian revolt erupted 15 months ago, triggering fears of a wider regional conflict.

Unal, the Foreign Ministry spokesman, said Turkey’s investigation showed that the plane had briefly strayed into Syrian airspace while on a routine mission to test Turkish radar systems. But the jet was immediately warned by the Turks, he said, and the missile strike came 15 minutes after the “brief violation,” when the plane was back in international airspace and was heading in “a different direction” than Syria.

It is not unusual for planes to briefly traverse the airspace of neighbors, and there were many steps Syria could have taken to notify the aircraft, he said, including communicating with Turkish authorities, attempting to reach the pilot and firing warning shots. None of those was done, he said.

His account, and one given by Turkish Foreign Minister Ahmet Davutoglu to Turkish state broadcaster TRT, differed from Syria’s claim that the shooting came after “an identified aerial target” was seen flying at “high speed and low altitude” toward the Syrian coast near the port of Latakia.

Intercepted Syrian communications suggest that the Syrians knew it was a Turkish plane and made a “deliberate” decision to shoot at it, Unal said. He said the Syrians fired “at least a couple” of surface-to-air missiles at the jet.

“The plane’s identity could be seen by all. It was not hiding anything,” Davutoglu said in the television interview. The plane was 13 nautical miles off the Syrian coast — a mile inside international waters — when it was hit, he said.

The incident signaled a new low in the once-close relationship between Ankara and Damascus, which had already deteriorated dramatically since a Turkish attempt to persuade President Bashar al-Assad to adopt political reforms failed last summer. Turkey is hosting the leadership of the rebel Free Syrian Army at a refugee camp in southern Turkey and recently joined in an effort to supply rebels inside Syria with arms and money, in collaboration with Saudi Arabia, Qatar and the United States.

Invoking Article 4 of the NATO charter is not considered as serious a step as Article 5, which requires members to spring to the defense of any ally that is under attack. But the fact that NATO is being drawn into the global debate on how to resolve the Syrian conundrum marks a new phase in an effort that has so far focused on U.N. diplomacy, said Andrew Tabler of the Washington Institute for Near East Policy.

“The mood and the tenor of all this changes,” he said.

After Turkey publicized its findings, other NATO allies weighed in with condemnations. British Foreign Secretary William Hague called the shooting “outrageous,” and said Britain was ready to pursue “robust action” at the U.N. Security Council.

Clinton said she would be consulting with U.S. partners over the incident, including the Security Council and Kofi Annan, the U.N. special envoy for Syria who is overseeing implementation of the U.N. peace plan — making it clear that she is not giving up on a diplomatic solution.

But another weekend of bloodshed inside Syria seemed only to underscore the failings of the U.N. effort. The Britain-based Syrian Observatory for Human Rights said it recorded 53 deaths on Sunday and 84 on Saturday, amid reports that government forces were stepping up an assault on the eastern city of Deir el-Zour and sustaining their bombardment of the central city of Homs.

Syria’s official SANA news agency said a record 112 members of the Syrian security forces had been buried Saturday and Sunday, an indication that the rebels are growing more effective in their counterattacks.

Russia, one of Syria’s staunchest allies, has repeatedly said it would use its veto to prevent any Security Council action that might open the door to military intervention. But Syria has now presented NATO with a pretext for involvement that could potentially bypass the United Nations, said Shaikh of the Brookings Doha Center.

“Assad has made a very big mistake,” he said. “He’s shown the very real dangers of this regime to its neighborhood.”