Turkey has shot down a Russian warplane.
This is the first time a Nato member country and Moscow have engaged during the crisis in Syria.
Reports out of Ankara and Moscow have provided conflicting accounts of the incident, which is understood to have occurred near the Turkish-Syrian border.
The Turkish military scrambled two F-16 fighter jets after a plane breached Turkish airspace in the province of Hatay at 9.20am on Tuesday morning.
The fighter pilots squawked a warning to the jet, telling it to leave ten times in five minutes.
The Russian jet was then shot down.
Russia’s defence ministry’s twitter account confirmed a Russian SU-24 had been shot down, but insisted the plane had never left Syrian airspace.
The defence ministry said: “At all times, the SU-24 was exclusively over the territory of Syria…
“The SU-24 was at 6000 metres and preliminary information suggests it was brought down by fire from the ground. The circumstances are being investigated.”
It is thought the pilots managed to eject from the plane.
Local media reports a pilot was captured by tribesmen in the area.
The downing of a Russian warplane by Turkey shows the “toxic cocktail” of dangers in the region, an expert has warned.
The combination of numerous international nations involved in the fight against Islamic State (IS) and the bloody civil war in Syria means tensions are high.
Turkey has claimed its F-16s shot down the Russian plane after it violated Turkish airspace and ignored repeated warnings.
But Moscow said the Su-24 jet was downed by artillery fire while on a bombing mission in Syria and had not strayed across the border.
The Russian defence ministry statement said: “We are looking into the circumstances of the crash of the Russian jet. The Ministry of Defence would like to stress that the plane was over the Syrian territory throughout the flight.”
Video footage of the incident shows a warplane on fire before crashing on a hill, and two crew members can be seen apparently parachuting to safety.
The Russian defence ministry said the pilots ejected but Moscow has had no further contact with them.
Middle East expert Shashank Joshi, from the Royal United Services Institute, said the skies over Syria and Turkey are an “incredibly crowded airspace”, with planes from both nations and members of the US-led coalition against IS – including the UK – operating.
Turkey, a Nato member, has already complained about Russian incursions into its skies and last month the alliance condemned the “unacceptable violations of Turkish airspace by Russian combat aircraft”.
Mr Joshi said: “The situation is dangerous because Russia is quite probably deliberately probing Turkish airspace both for military reasons and political reasons.”
The Russians will be testing the military responses of the Nato member, but also carrying out the same “psychological intimidation” tactics used in the Baltic and North Atlantic, he suggested.
The combination of the crowded airspace, Russian probing tactics and the diplomatic tensions create a “real toxic cocktail that can easily erupt into crisis”, he warned.
Ankara will be “furious” at the incursion and Russia can expect Nato to strike a “tough” note, but behind the scenes there will be intense diplomatic efforts to calm tensions.
But if Moscow responds in a provocative way, there is a risk of the crisis escalating.
Mr Joshi warned: “These things always proceed in a very unpredictable fashion. We have seen how conflicts can begin when there are large alliances.”
Russia’s participation in the Syrian peace process talks in Vienna, the co-operation on the UN Security Council resolution and meetings between Russian President Vladimir Putin and Nato leaders provided signs of a renewed diplomatic engagement between Moscow and the West in recent weeks.
French President Francois Hollande will meet Mr Putin on Thursday and Russia has offered co-operation in the fight against IS following the atrocities in Paris and the downing of a Russian passenger jet in Egypt.
Video emerged later in the day which is thought to show one of the pilots, dead.
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