Russia has sent two tank-landing ships and additional aircraft to Syria in the past day or so and has deployed a small number of forces there, U.S. officials said on Wednesday, in the latest signs of a military buildup that has put Washington on edge.
The two U.S. officials, who spoke on condition of anonymity, said the intent of Russia's military moves in Syria remained unclear.
U.S. officials have not ruled out the possibility that Moscow may be laying the groundwork for an air combat role in Syria's conflict to bolster Syrian President Bashar al-Assad.
Assad, a longtime Russian ally, has seen the area he controls whittled down to a fifth or less of Syria's territory after more than four years of grinding civil war.
An official with the Russian Defense Ministry declined to comment.
A traditional arms supplier to Damascus, Moscow has supported Assad throughout the war that has fractured Syria and has said it strongly opposes Islamic State, a militant group that is also the target of a U.S.-led air campaign.
One of the U.S. officials said initial indications suggested the focus was on preparing an airfield near the port city of Latakia, a stronghold of Assad.
The officials estimated that dozens of the naval infantry forces had recently arrived at the airfield, possibly to help provide security.
Reuters has previously reported on the transport of prefabricated housing units for hundreds of people to the Syrian airfield. The two officials said additional Russian prefabricated housing units had arrived as well.
The Russian tank landing ships arrived in the past day or so at the port city of Tartus, just south of Latakia, the officials said, without providing information on the cargo. Additional cargo aircraft had also arrived at the airfield near Latakia.
The United States and Russia have long been at loggerheads over Syria. Russia has backed Assad, while the United States advocates a political transition to end his rule.
On Tuesday, the Pentagon said Russian support for Assad risked "further escalating the conflict."
"If there are further efforts to support the Assad regime from a military standpoint on the part of the Russians, we would again see that with concern," Pentagon spokesman Peter Cook told a news briefing.