According to a number of Lebanese sources, Russian forces have begun participating in military operations in Syria in support of government troops.
The unidentified sources, speaking to Reuters on Wednesday, gave the most forthright account yet from the region of what the United States fears is a deepening Russian military role in Syria's civil war, though one of the Lebanese sources said the number of Russians involved so far was small.
U.S. officials said Russia sent two tank landing ships and additional cargo aircraft to Syria in the past day or so and deployed a small number of naval infantry forces.
The U.S. officials, who also spoke on condition of anonymity, said the intent of Russia's military moves in Syria was unclear. One suggested the focus may be on preparing an airfield near the port city of Latakia, a stronghold of Syrian President Bashar al-Assad.
U.S. officials have not ruled out the possibility that Russia may want to use the airfield for air combat missions.
U.S. Secretary of State John Kerry spoke to his Russian counterpart for the second time in four days to express concern over reports of Russian military activities in Syria, warning that it could fan more violence.
The White House said it was closely monitoring the situation.
Russia says the Syrian government must be incorporated into a shared global fight against Islamic State, the Islamist group that has taken over large parts of Syria and Iraq.
Assad's forces have faced big setbacks on the battlefield in a four-year-old multi-sided civil war that has killed 250,000 people and driven half of Syria's 23 million people from their homes.
Syrian troops pulled out of a major air base last Wednesday, and a monitoring group said this meant government soldiers were no longer present at all in Idlib province, most of which slipped from government control earlier this year.
Moscow confirmed it had "experts" on the ground in Syria, its long-time ally in the Middle East.