Washington (AFP) - Russia is pressing ahead with its military buildup in Syria, US officials said on Wednesday, though Moscow's intentions in the war-ravaged nation remain unclear.

Aerial imagery shows Russia is focusing on Bassel al-Assad International Airport, south of Latakia on Syria's Mediterranean coast, and on the Russian naval facility in Tartus.

One US official, speaking on condition of anonymity, told AFP that two tank-landing ships have recently arrived at Tartus and about a dozen Russian armored personnel carriers are now at the Bassel al-Assad airport, named after Syrian President Bashar al-Assad's older brother.

The official said dozens of Russian naval infantry had arrived in Syria, but their role was likely to protect incoming military hardware rather than a boots-on-the-ground deployment.

"We've seen scores there," the official said. "There certainly are some troops there."

In addition, another giant Antonov-124 Condor military transporter had flown into the airport, bringing the total number of transport flights to at least four in recent days.

AFP reported Tuesday that Russia has also installed modular housing units -- enough for "hundreds" of people -- at the airport, as well as portable air traffic control equipment.

The developments further complicate the deadly crisis in Syria, where the conflict has claimed nearly 250,000 lives since 2011 and triggered a massive outflow of refugees -- many of whom are fleeing to Europe.

Moscow insists it is bringing humanitarian aid to a stricken nation, but the United States, which is attacking Islamic State jihadists in Syria, is worried Russia is preparing to assist militarily and strike rebel groups to help prop up long-time ally Assad.

"There's not consensus in the intelligence community about what it means," another US official said, again speaking on condition of anonymity.

"You have to take what Russia says with a grain of salt because they haven't always been transparent and honest about their intentions," the official added, referring to Russia's continued denials that it is supporting separatist rebels in eastern Ukraine.

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- 'Unconscionable' to support Assad -

Syria has denied reports of increased military activity by Russian troops on its soil and Russia insists any arms deliveries are in keeping with traditional links to the Assad regime.

White House deputy press secretary Eric Schultz on Wednesday said the United States would welcome any Russian contributions to the fight against the Islamic State group.

But "it would be unconscionable for any party, including the Russians, to provide any support to the Assad regime," Schultz told reporters.

Despite the buildup of military equipment, officials noted there was no immediate indication Russia had sent in any ordnance or heavy weaponry. A US military official said "under 50" Russian troops had arrived in Syria in recent days.

Russia is facing logistical difficulties in getting its hardware to Syria by air.

Bulgaria said it refused permission late last week to an unspecified number of Russian aircraft to cross its airspace between September 1 and 24, angering Moscow.

But on Wednesday, Bulgaria said it would allow Syria-bound Russian planes over its airspace if Moscow agrees to let it inspect the cargo.

In the event Bulgaria closes its skies to Russian overflights, Moscow could route its planes over Iran and then Iraq.

When questioned whether the United States had asked Iraq to block Russian overflights, State Department spokesman John Kirby said America wanted regional partners -- including Iraq -- to ask some "pretty tough questions" about Russia's intent in Syria.