Russia's Defense Ministry says a Russian military adviser has been killed by mortar fire in Syria.The ministry said in a statement that the officer was fatally wounded Monday by mortar shelling from the Daesh group. Wednesday's statement, carried by Russian news agencies, said the officer was helping train the Syrian regime military in using Russian weapons. It didn't identify the officer or specify where he died.The officer's death is the third combat casualty the Russian military has suffered since it launched its air campaign in Syria four months ago.