Washington (AFP) - At least three Russian military transport planes have landed in Syria in recent days, US officials said Tuesday, as Washington worries about the sort of assistance Moscow is providing to Damascus.

The aircraft have landed at the airport in Latakia on Syria's Mediterranean coast over the past several days, US officials told AFP on condition of anonymity.

Two of the aircraft were giant Antonov-124 Condor planes and a third was a passenger flight, one of the officials said.

The Russians have installed modular housing units -- enough for "hundreds" of people -- at the airport, as well as portable air traffic control equipment, the official noted.

"All of this seems to be suggesting that Russia is planning to do some sort of forward air-operating hub out of this airfield," the official said.

Washington has expressed concern following reports suggesting Moscow may be boosting military support to Syrian President Bashar al-Assad and had sent a military advance team to the war-torn country.

"Our concern would be that any effort to bolster the Assad regime right now would potentially be destabilizing," Pentagon press secretary Peter Cook said Tuesday.

Syria has denied reports of increased military activity by Russian troops on its soil.

US Secretary of State John Kerry called his Russian counterpart Sergei Lavrov over the weekend to warn of the dangers of stoking a conflict which has already cost nearly 250,000 lives.

Russia rejects the charge, insisting that any deliveries are in keeping with its traditional links to long-time ally Syrian President Bashar al-Assad.

Bulgaria has refused permission for Russian aircraft to enter its airspace, and Greece said Tuesday that Washington had asked it to also deny Russian overflights.