ANKARA, Turkey (AFP) – NATO member Turkey shot down a Russian fighter jet on the Syrian border on Tuesday, threatening a major spike in tensions between two key protagonists in the four-year Syria civil war.

The Turkish army said the plane was shot down by two Turkish F-16s after violating Turkish airspace 10 times within a five-minute period.

With a major diplomatic crisis looming between two states on opposing sides in the Syria conflict, Russia insisted the jet was all the time inside Syrian airspace and condemned the downing as “a very serious incident”.

Turkish media said one pilot had been captured by rebel forces in Syria after both ejected by parachute while Syrian opposition sources said one was dead and another missing.

The fighter jet exploded in mid-air, crashing in a fireball onto a mountain on the Syrian side of the border, television pictures showed.

The presence of aircraft from Russia, the United States, France, Turkey and a clutch of Gulf states in Syrian skies had long raised fears of an incident that could quickly escalate into a major diplomatic and military crisis.

“A Russian Su-24 plane was downed under the rules of engagement because it violated Turkish airspace despite the warnings,” the Turkish presidency said.

Turkey has summoned the Russian envoy to Ankara over the incident, which comes on the eve of a visit by Russian Foreign Minister Sergei Lavrov to the country.

Russia confirmed that one of its planes had been downed at an altitude of 6,000 metres (20,000 feet) but said it appeared to have been shot down from the ground.

“Presumably as a result of firing from the ground, an Su-24 plane of the Russian forces crashed in the Syrian Arab Republic,” Russian news agencies quoted the defence ministry as saying.

“It is a very serious incident,” Kremlin spokesman Dmitry Peskov told reporters.

The Turkish army said that the downing took place over the Yayladagi district of Turkey’s Hatay province on the border with Syria.

“The plane violated Turkish air space 10 times in five minutes despite warnings,” the army said in a statement, adding it was shot down by at 0724 GMT “according to the rules of engagement”.

Reports said two pilots had ejected from the plane and Turkish television pictures showed two white parachutes descending to the ground.

Their fate was not certain.

CNN-Turk said Syrian Turkmen forces fighting the Russian-backed regime of President Bashar al-Assad had captured one pilot.

Syrian opposition sources meanwhile told AFP one pilot was dead, the second missing.

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 men.

The incident came as Russian and Syrian jets are waging a heavy bombing campaign against targets in northern Syria.

Turkey has expressed anger at the operation, saying it is aimed at buttressing the Syrian regime and has displaced thousands of Turkmen Syrians, an ethnic minority in the area and strong allies of Ankara.

Russia however insists the air strikes are aimed against Islamic State jihadists.

At Ankara’s request, NATO allies will hold an “extraordinary” meeting at 1600 GMT to discuss the incident, an alliance official said.

“NATO is monitoring the situation closely. We are in contact with Turkish authorities.”

Russian fighter jets entered Turkish airspace in two separate incidents in October, prompting Ankara to summon the Russian ambassador twice to protest both violations. 

Turkey and Russia have long been at loggerheads over the Syrian conflict, with Ankara seeking Assad’s overthrow while Moscow does everything to keep him in power. 

The Turkish military in October also shot down a Russian-made drone that had entered its airspace. But Moscow denied the drone belonged to its forces. 

It remains to be seen what action Turkey could call for at NATO.

Turkey in July invoked NATO’s rarely-used article four — which allows any member to request a meeting of all 28 NATO ambassadors — over its campaign against Kurdish rebels.

Lavrov is due to visit Turkey on Wednesday in a bid to smooth ties and find a joint approach to finding peace in Syria.

Along with Saudi Arabia and the United States, Turkey and Russia are taking part in talks in Vienna that aim to narrow differences on the Syria conflict and have taken on an extra importance after the Paris attacks.

A Turkish foreign ministry official told AFP Lavrov’s visit would go ahead as planned:  “There is no change in the programme.”