By Matt Spetalnick and Will Dunham WASHINGTON (Reuters) - 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 a**nymity, said Moscow?s exact intentions remained unclear but that Kerry called Russian Foreign Minister Sergei Lavrov to leave ** doubt of the U.S. position.