United States Defence Secretary Chuck Hegel has described Russia's reported decision to send advanced anti-ship missiles to Syria's government as unfortunate and ill-timed.
Joint Chiefs of Staff chair General Martin Dempsey said the shipment is ill-timed and very unfortunate.
Without confirming details, Foreign Minister Sergei Lavrov said the Russian consignment did not break any international rules.
According to IHS Jane's International Defense Review, the Yakhont missiles have a range of about 300km and carry a 200kg warhead.
Russia is one of Syria's few remaining allies. The Wall Street Journal reports Moscow has deployed at least a dozen warships on patrol near the Russian naval base in the Syrian city of Tartus.