Turkish fighter jets shot down an unidentified drone which entered its air space near the Syrian border, with the US military saying "all indications" showed that the unmanned vehicle was Russian.
If confirmed, the presence of a Russian craft in the air space of the key NATO member would further ratchet up tensions over Moscow's behaviour in Syria. Russia denied it was to blame, despite repeated previous violations.
The Turkish army said that the craft had been warned three times by Turkish planes but had maintained course.
It was then "downed by fire from our aircraft on patrol, according to the rules of engagement."
The statement did not say if the downed aircraft was manned or a drone. But a Turkish official told AFP in Ankara it was an "unmanned aircraft".
The army warned the air force would "decisively" implement Turkey's rules of engagement.
The US military believes the drone downed by Turkey was Russian, a US official said in Washington.
"All indications are, yes, it is a Russian drone," the official said, speaking on condition of anonymity. The official added there were no reports of the Syrian army using the type of drone that was shot down.
Television pictures showed the military examining the crash site by the village of Deliosman in the southern Kilis region on the Syrian border.
The state-run Anatolia news agency published pictures of what it said was the downed craft lying upside down in the grass, showing it to be a small vehicle just a few feet in length.
It appeared to be a drone used for reconnaissance operations.
Analysts said that the drone was similar to drones used by pro-Russian separatist forces in Ukraine, some of which had been shot down.
"A good working assumption is that it's Russian," said Nick de Larrinaga, Europe editor of IHS Jane's Defence Weekly, adding it was nonetheless different to a Russian Orlan 10 drone.
But Russia's military on Friday said all its planes in Syria had returned safely to base and its drones were "working as normal".
"All the Russian planes in Syria have returned to the Hmeimim air base after completing their tasks," defence ministry spokesman Igor Konashenkov told Russian news agencies.
"Russian unmanned aerial vehicles monitoring the situation on the territory of Syria and carrying out air reconnaissance are working as normal."
Turkish Foreign Minister Feridun Sinirlioglu said Ankara had still not identified the drone. "We will announce once the nationality is clarified," he said.
He said the drone had fallen three kilometres (1.85 miles) inside Turkish territory.
Turkey had earlier this month bitterly complained about two violations of its air space by Russian warplanes operating in Syria.
A NATO official said Turkey had not asked for NATO assistance or consultations over the downing.
"We understand that Turkish authorities are investigating the origin of the unidentified drone that was shot down," said the official in Brussels.
Turkish Prime Minister Ahmet Davutoglu had warned on October 5 after the Russian air space violation that Ankara would activate military rules of engagement irrespective of who violates its air space.
"The Turkish armed forces are clearly instructed. Even it is a flying bird it will be intercepted," Davutoglu said.
Turkey supports the moderate opposition in Syria and has been infuriated by Moscow's bombing campaign to support President Bashar al-Assad.
Sinirlioglu said that the Russian air strikes had not been aimed at striking at ISIS jihadists and were simply seeking to prop up the regime.
"This is not a method that will ensure Syria's unity. Nothing will emerge other than protracting existing chaos and war and more human losses," he said.
Sinirlioglu said the Russian strikes near the Turkish border could lead to dangerous situations. "This is a serious issue that could lead to accidents."