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"As darkness fell, the Hmeimim air base was subjected to sudden mortar shelling from a mobile group of militants. As a result of the shelling, two servicemen were killed," the defence ministry said in a statement to Russian agencies.

The Kommersant business daily reported seven military planes had been "practically destroyed" in the attack, citing two military-diplomatic sources, but the ministry said the report was "fake". 

The news comes a day after the ministry announced a Russian helicopter crash in Syria that killed two pilots following a technical fault, also on December 31. 

With four fatalities this is one of the deadliest single days for the Russian army in Syria since it entered the conflict and brings the total number of officially reported losses to 44. 

Security around the Hmeimim base was being stepped up after the mortar attack, the defence ministry said.

"The Russian air force in Syria is combat-ready and continues to fulfill all intended tasks," the statement added.

Russian President Vladimir Putin made a surprise visit to Syria last month where he ordered the start of a pullout of Russian troops, saying their task in the war-torn country had been largely completed.

Three battalions of military police and officers of the Russian Center for Reconciliation would remain in Syria, as well as two Russian bases, Defence Minister Sergei Shoigu said on a subsequent trip. 

Russia became involved in the multi-front conflict in September 2015, when it began an aerial campaign in support of President Bashar al-Assad's military. 

Moscow acknowledged in recent months that its special forces are also active on the ground in the offensive against the Islamic State group.