Russia on Tuesday described the downing of one of its fighter jets on the Turkey-Syria border as a "very serious incident", insisting its plane had not crossed into Turkish territory.
NATO-member Turkey said the plane had violated Turkish airspace 10 times within a five minute period and was shot down by two Turkish F-16s.
Kremlin spokesman Dmitry Peskov said the downing was a "very serious incident" but insisted it was too early to make any conclusive statement.
"It would be wrong to make some assessments, proposals or announcements when we don't have the full picture," Peskov told reporters.
It is the first time a Russian military plane has been downed since Moscow began a bombing campaign in Syria on September 30 at the request of long-standing ally President Bashar al-Assad.
The defence ministry confirmed that the downed Sukhoi plane was Russian but said it appeared the aircraft had been shot "from the ground" despite Turkey saying its fighter jets had hit the plane.
The ministry said the fate of the aircraft's two pilots was not yet clear but that preliminary reports suggested they had ejected.
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"Presumably as a result of firing from the ground, an Su-24 plane of the Russian forces crashed in the Syrian Arab Republic," the defence ministry said in a statement.
"The plane was at an altitude of 6,000 metres. The fate of the pilots is being determined," the statement said.
"Throughout its flight the plane remained exclusively over Syrian territory."
The fate of the pilots was not immediately clear although rebel and opposition activist sources in Syria told AFP that one was dead and the second missing.
President Vladimir Putin is likely to discuss the incident with Jordanian King Abdullah II at a meeting in the Black Sea resort of Sochi Wednesday, Peskov added.
Moscow's intervention in Syria has strained relations with Turkey -- a fierce opponent of Assad -- with Ankara summoning Russia's ambassador last week after Moscow's warplanes bombed Syrian territory "very close" to the Turkish border.
Turkish authorities have already summoned the Russian ambassador several times since Moscow launched its Syria campaign over alleged air space violations.
Ankara has also warned Moscow against supplying arms and support for Syrian Kurdish forces fighting the Islamic State group in Syria.
Russian Foreign Minister Sergei Lavrov is set to hold talks with his Turkish counterpart Feridun Sinirlioglu in Istanbul on Wednesday at a meeting planned before the plane downing.