A Russian military transport plane crashed in Syria on Tuesday, killing all 39 people on board, Russian officials said, an incident that sharply raised the death toll from the Kremlin's intervention in the Syrian war.

President Vladimir Putin, who is running for re-election later this month, declared in December that the Russian mission in Syria was largely completed, but casualties continue to mount.

The Kremlin said in a statement that the plane, a Soviet-designed An-26, crashed at Russia's Hmeymim air base in Latakia Province. The cause may have been a technical malfunction, the Kremlin said.