Two Russian planes carrying humanitarian aid landed in Syria on Saturday, state media said, amid reports that Moscow is beefing up military support to its ally Damascus.

"Two Russian planes arrived today at the Latakia Martyr Bassil al-Assad international airport carrying 80 tonnes of humanitarian aid provided by Russia," state news reported.

Coastal Latakia province is a stronghold of President Bashar al-Assad and home to his ancestral village.

The reported aid delivery comes as Washington expressed concern about an alleged Russian military build-up in Syria.

US officials say Russia has sent ships, armoured personnel carriers and naval infantry to the country in recent weeks.

And on Friday, Cypriot officials said Russia had issued an alert for Cyprus to divert aircraft next week because it is planning military exercises off Syria's coast.

Also Friday, US President Barack Obama said Russia's decision to send military advisors and equipment to bolster Assad was only extending a strategy "doomed to failure."

"The strategy that they are pursuing right now, doubling down on Assad, I think is a big mistake," Obama said.

Moscow and Syria have denied any Russian military build-up, but Russian Foreign Minister Sergei Lavrov said Thursday that Damascus would receive additional help if it requested it.

"We helped, are continuing to help and will help the Syrian government when it comes to supplying the Syrian army with everything it needs," he said.

"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."

Moscow has been a staunch ally of the Assad government throughout the uprising that began in March 2011 and later descended into a civil war.

It maintains a naval base in Tartus province, south of Latakia province.

Syrian media has reported Russian deliveries of humanitarian aid sporadically throughout the conflict, but Saturday's report follows specific claims about a Russian military build-up.

US officials this week said two tank-landing ships had arrived recently at the Tartus base, but most of the apparent build-up was focused on the Bassil al-Assad airport.

They said at least four transport flights had arrived in recent days, with dozens of Russian naval infantry also coming in.

Russia has also reportedly installed temporary housing sufficient for "hundreds of people" at the airport, along with portable air traffic control equipment.

More than 240,000 people have been killed in Syria since the conflict began, with Assad's government losing increasingly large parts of the territory to rebels or jihadist forces like the Islamic State group.

The regime has relied for support on a few staunch allies, particularly Russia and Iran.