Secretary of State John Kerry told his Russian counterpart on Saturday the United States was deeply concerned about reports that Moscow was moving toward a major military build-up in Syria widely seen as aimed at bolstering President Bashar al-Assad.
U.S. authorities have detected "worrisome preparatory steps," including transport of prefabricated housing units for hundreds of people to a Syrian airfield, that could signal that Russia is readying deployment of heavy military assets there, a senior U.S. official told Reuters.
The official, speaking on condition of anonymity, said Moscow's exact intentions remained unclear but that Kerry called Russian Foreign Minister Sergei Lavrov to leave no doubt of the U.S. position.
The United States and Russia have long been at loggerheads over Syria.
Russia has backed Assad.
The United States advocates a political transition to end his rule.