NATO member Turkey has shot down a Russian fighter jet on the Syrian border.
The Turkish presidency said in a statement on Tuesday (local time) that the plane was a Russian Su-24 fighter jet, while Turkish media said one pilot had been captured by rebel forces in Syria.
Moscow confirmed that one of its planes had been shot down, but said the pilots' fate remained unclear.
The Turkish army said that the plane had violated Turkish airspace 10 times within a five minute period and was shot down by two Turkish F-16s.
However Russian authorities insisted they could prove the jet had not left Syrian air space.
"A Russian Su-24 plane was downed under the rules of engagement because it violated the Turkish airspace despite the warnings," the Turkish presidency said.
Reports said two pilots had ejected from the plane and Turkish television pictures showed two white parachutes descending to the ground.
The CNN-Turk channel said Syrian Turkmen forces fighting the Russian-backed regime of President Bashar al-Assad captured one pilot.
Russia meanwhile confirmed that one of its planes had been shot down.
"Presumably as a result of firing, an Su-24 plane of the Russian forces crashed in the Syrian Arab Republic," Russian news agencies quoted the defence ministry as saying.
The Russian ministry said the fate of the pilots was not yet clear.
Turkey's Dogan news agency broadcast footage of what it said was Russian helicopters flying over Syrian territory in an apparent search for the lost pilots.
The fighter jet exploded in the air and the fireball fell on a mountain on the Syrian side of the border, television pictures showed.
Footage posted by the state-run Anadolu news agency showed plumes of smoke rising behind a mountain a few kilometres from the Turkish border.
Turkish reports said the incident happened in the border area between Turkey's southern Hatay province and an area in northwest Syria populated by the Turkic-speaking Turkmen minority.
Amid fears of a diplomatic crisis, key NATO member Turkey said it would take the issue to the United Nations and NATO.
"Necessary initiatives will be taken at NATO, UN and at the level of countries concerned by the foreign ministry upon instructions from Mr Prime Minister," a statement from Prime Minister Ahmet Davutoglu's office said.