BEIRUT, Lebanon — Turkey announced Friday that Syrian forces had shot down a Turkish warplane with two crew members over the Mediterranean, a potentially ominous turn for the worse in relations already frayed because of Turkey’s support for Syrian rebels fighting to overthrow President Bashar al-Assad.

The Syrians later acknowledged their antiaircraft gunners had downed the plane, contending it had violated Syrian airspace.
Turkey’s announcement, from the office of Prime Minister Recep Tayyip Erdogan, came after an emergency security summit by Turkish officials in Ankara over the fate of the plane, which had been reported missing hours earlier amid conflicting reports over whether it had crashed or been downed intentionally.
The measured wording of the announcement suggested that Turkey had not yet concluded the Syrian action was provocative, and it acknowledged that Syrian rescue teams were cooperating in trying to locate the aircraft and crew. But the statement also left open the possibility that Turkey, a NATO member, would respond militarily, an outcome that could further complicate and widen the Syrian conflict.
“As the result of the data provided by our related institutions and the research jointly conducted with Syria, it was understood that our plane has been downed by Syria,” said the statement from Mr. Erdogan’s office. “Turkey, after the incident is fully enlightened, will lay forward its attitude and take necessary steps.”
It was the first time since Syrian-Turkish relations began to deteriorate last year that the Syrian military had shot down a Turkish military aircraft.
The plane, identified by Turkish and Western news agencies as an American-made F-4 Phantom, went down in the Mediterranean near the southern coast of Turkey’s Hatay Province, which borders Syria’s Latakia Province.
The Syrian Defense Ministry said later in a statement carried by the official Syrian Arab News Agency that its antiaircraft gunners had hit an unidentified aircraft flying at “very low altitude and at high speed” over Syria’s territorial waters less than a mile offshore, causing it to crash into the sea near the village of Om al-Tuyour. The Syrian statement said it was later determined that the aircraft was a “Turkish military plane that entered Syrian airspace and was dealt with according to laws observed in such cases.”
Mr. Erdogan is one of the most strident critics of Syria’s harsh repression of its anti-government uprising, now in its 16th month. Turkey has allowed more than 32,000 Syrian refugees to seek sanctuary and has permitted Syria’s opposition forces and exile dissidents to organize and funnel supplies from Turkey over the 550-mile border into Syria.
Turkey is also a leading member of the “Friends of Syria” group of countries, including the United States, that has been pushing for the ouster of Mr. Assad. Last month, Turkey joined the United States and European countries in expelling Syrian diplomats in response to evidence that Mr. Assad’s forces had massacred civilians in western Syria.
The new rupture in Turkey-Syria relations came as reports of a possible new mass killing emerged Friday in northern Syria’s Aleppo Province.
Opposition activists said those killings, in the village of Daret Azzeh, were the outcome of a military skirmish in which soldiers of the Free Syrian Army, the main armed rebel force, carried out a surprise attack on a group that included suspected members of the shabiha, the feared pro-government militia accused by the opposition of complicity in numerous killings, detentions and torture. At least 25 men were shot to death.
“The armed opposition in the area ambushed a number of cars,” said the Syrian Observatory for Human Rights, an antigovernment group based in Britain with networks of contacts inside Syria.
A video that the Observatory posted online from activists, said to have been recorded in Daret Azzeh, showed carnage, with bloodied corpses piled around a white pickup truck riddled with bullet holes. Many corpses were clothed in military fatigues or black clothes, a trademark of the shabiha.
The official Syrian Arab News Agency’s account of the Daret Azzeh killings attributed them to armed terrorist gangs, the standard government description for all opposition forces, and said the killers had kidnapped and tortured the victims.
Dalal Mawad reported from Beirut, and Rick Gladstone from New York. Reporting was contributed by Sebnem Arsu from Istanbul, Turkey, Hwaida Saad from Beirut, Nick Cumming-Bruce from Geneva and Ellen Barry from Moscow.