Syria has said it shot down a Turkish military plane that entered Syrian air space, while Turkey vowed to "determinedly take necessary steps" in response.
It was the most clear and dramatic escalation in tensions between the two countries, which used to be allies before the Syrian revolt began in March 2011.
Turkey has become one of the strongest critics of the Syrian regime's brutal response to the country's uprising.
On Friday night, Syria's state-run news agency, Sana, said the military spotted an "unidentified aerial target" that was flying at a low altitude and at a high speed.
"The Syrian anti-air defences counteracted with anti-aircraft artillery, hitting it directly," Sana said. "The target turned out to be a Turkish military plane that entered Syrian airspace and was dealt with according to laws observed in such cases."
Turkey issued a statement following a two-hour security meeting led by Turkish Prime Minister Recep Tayyip Erdogan, saying Syrian forces downed the plane and that the two Turkish pilots remain missing.
It said Turkey "will determinedly take necessary steps" in response, without saying what those actions would be. "Following the evaluation of data provided by our related institutions and the findings of the joint search and rescue efforts with Syria, it is understood that our plane was downed by Syria," the statement said, without providing other details.
Relations between Turkey and Syria were already tense before the downing of the F4 plane. Turkey has joined nations such as the US in saying that Syrian President Bashar Assad should step down because of the regime's brutal suppression of the uprising in his country. Turkey has also set up refugee camps on its border for more than 32,000 Syrians who have fled the fighting.
Syria and Turkey have expelled each other's ambassadors and Syria has accused Turkey of supporting Syrian opposition and even allowing Syrian rebels to operate out of Turkish soil. Turkey strongly denies the allegations.
After a cross-border shooting by Syrian forces in April, Turkey said it would not tolerate any action that it deemed violates its security. The firing had left two refugees dead at a camp near the town of Kilis just inside Turkey.
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