Russia's recent movements near Syria's city of Latakia suggest that Moscow plans to establish a "forward air operating base" there, the US has said.
Pentagon spokesman Jeff Davis said there had been a steady flow of people and equipment in the coastal area.
Russia says military equipment is being sent to Syria to help the government combat the so-called Islamic State.
Moscow has been a key ally of President Assad during Syria's bloody civil war, which began in 2011.
'No additional steps'
"We have seen indications in recent days that Russia has moved people and things into the area around Latakia and the air base there that suggests that it intends to establish some sort of a forward air operating base," Mr Davis said on Monday.
He added that the US was concerned that Russian military moves could come into conflict with US and coalition air strikes that were being conducted in Syria against IS.
US officials also fear that the stepped-up Russian support will fortify the regime of President Bashar al-Assad at a time when it has been losing on the battlefield and will complicate recent efforts to find a political solution to the conflict, the BBC's Barbara Plett Usher in New York reports.
The US comments come just days after Syrian state media reported that two Russian cargo planes with 80 tonnes of humanitarian aid had landed in Syria.
The planes reportedly flew into an air base near Latakia - a stronghold of President Assad.
Russian Defence Ministry spokesman Maj Gen Igor Konashenkov said the plane was carrying materials for setting up a tent camp for refugees.
The Kremlin has dismissed any military build-up, although Foreign Minister Sergei Lavrov said last week that Russia would send more help to Syria if requested.
"Russia is sending planes to Syria with both military equipment in accordance with current contracts and humanitarian aid," he said. "Russia is not taking any additional steps."
US officials have said Russia may be giving President Assad extra military support because he has suffered substantial territorial losses to rebels.
The Syrian government recently lost control of a key air base in the north-eastern province of Idlib.
Earlier this month, Bulgaria refused to allow Russian aircraft to cross its air space amid fears that Russia was sending extra military support to Syria.
Russia later said it had been given permission to fly over Iranian territory en route to Syria.
Last week, officials in Washington quoted by Reuters said Moscow had sent more aircraft and two tank landing ships to Russia's naval base in the Syrian coastal city of Tartus. They also said a small number of naval infantry forces had been deployed.
While Russia has backed the Syrian government and provided it with arms, the US wants to see President Assad removed.
The war between President Assad's regime and various rebel groups has so far killed at least 240,000 people and displaced millions.