The Russian military will respond to a U.S. strike on Syria, targeting any missiles and launchers involved in such an attack, the RIA news agency reported on Tuesday.
RIA cited Valery Gerasimov, head of the General Staff, as saying that Moscow would retaliate if the lives of Russian servicemen in Syria were threatened.
The head of Russia's military General Staff and the head of the U.S. Joint Chiefs of Staff discussed the Syrian crisis, including the situation in besieged eastern Ghouta, the Russian defence ministry also said on Tuesday.
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The two men agreed in their telephone conversation to continue bilateral contacts, it added.
Russia backs Syrian President Bashar al-Assad in that country's civil war, while the United States has backed rebel groups opposed to him.
Over 300 people have left Syria's eastern Ghouta district since a humanitarian corridor opened there, RIA news agency cited a representative for Russia's ceasefire monitoring centre in Syria as saying on Wednesday.
"Since the humanitarian corridor started working in the region of eastern Ghouta, more than 300 people have left, the majority of these people left in the last few days," RIA quoted Major General Vladimir Zolotukhin as saying.