Turkey's government has called an emergency security meeting amid reports that one of its fighter jets was shot down by Syrian security forces.
The Turkish military lost contact with an F-4 Phantom over the Mediterranean Sea, south-west of Hatay province.
Prime Minister Recep Tayyip Erdogan has told a news conference there is still no confirmation of what brought the jet down, nor of the fate of its two crew.
He was earlier quoted as saying: "The other side have expressed regret".
Relations between Turkey and Syria, once close allies, have deteriorated sharply since the uprising against Syrian President Bashar al-Assad began in March 2011.'Syrian waters'
Witnesses in the Syrian coastal city of Latakia meanwhile told BBC Arabic that Syrian air defences had shot down an unidentified aircraft near the town of Ras al-Basit.
Lebanon's al-Manar television channel - controlled by Lebanon's Shia Hezbollah movement, an ally of the Syrian government - also reported that Syrian security sources had said that "Syrian air defences shot down a Turkish warplane and hit another in Syrian airspace".