ANKARA (AFP, REUTERS) - The Turkish military on Friday (Oct 16) said it had downed an "air vehicle" of unknown origin which had violated its air space close to the Syrian border.

The 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.

It warned the air force would "decisively" implement Turkey's rules of engagement.

NTV television, without citing its sources, said the object was a drone, and it had fallen 3km inside Turkish territory.

Television pictures showed the military examining the crash site. The location was not specified.

The Russian defence ministry said on Friday (Oct 16) all its planes in Syria had safely returned to base and all its drones were operating "as planned", the RIA Novosti news agency reported.

The ministry was commenting on the reports that Turkish warplanes shot down a drone in Turkish air space near the Syrian border on Friday (Oct 16).

But the United States suspects the plane that Turkey shot down was a Russian drone aircraft, a US official told Reuters, cautioning that the information was still preliminary. The official, who spoke on condition of anonymity, declined to provide further details.

Turkey had earlier this month bitterly complained about two violations of its air space by Russian warplanes operating in Syria.

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.

"Our rules of engagement are clear whoever violates our air space," Mr Davutoglu said. "The Turkish armed forces are clearly instructed. Even it is a flying bird it will be intercepted," he added.

Turkey supports the moderate opposition in Syria and has been infuriated by Moscow's bombing campaign to support President Bashar al-Assad.