Russia is pressing ahead with its military build-up in Syria, US officials say, 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 armoured 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.
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.