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.<br />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.<br />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.<br />The United States and Russia have long been at loggerheads over Syria.<br />Russia has backed Assad.<br />The United States advocates a political transition to end his rule.
