Washington (AFP) - Russia has deployed 28 combat planes in Syria, US officials said Monday, confirming the latest move in Moscow's increasing military presence in the war-torn nation.

Washington in recent weeks has expressed growing concern over Russia's moves to support Syrian President Bashar al-Assad and warned that militarily backing his regime risks further hampering efforts at bringing peace.

Experts said the buildup is likely a prelude to military action.

"There are 28 fighter and bomber aircraft" at an airfield in the western Syrian province of Latakia, one of the officials told AFP, speaking on condition of anonymity.

A second official, also speaking on condition of anonymity, confirmed the figure and added there were also about 20 Russian combat and transport helicopters at the base.

That official also said Russia was operating drones over Syria, but did not give additional details.

According to the officials, Russia has sent 12 SU-24 attack aircraft, 12 SU-25 ground attack aircraft and four Flanker fighter jets.

Analyst Jeffrey White of the Washington Institute for Near East Policy said: "They are not going to sit around and defend the airfield or maybe even the province of Latakia.

"This kind of aircraft suggests that the Russians intend to exert their combat power outside of Latakia in an offensive role."

Moscow has been on a diplomatic push to get the coalition of Western and regional powers fighting the Islamic State group in Syria to join forces with Assad against the jihadists.

US Defense Secretary Ashton Carter spoke with his Russian counterpart Sergei Shoigu on Friday, ending an 18-month freeze in military relations triggered by NATO anger over Moscow's role in the Ukraine crisis.

They agreed to continue discussions, which are crucial to lessen the risk of incidents involving coalition forces and Russian forces operating in the same air space.

The US-led coalition is carrying out almost daily strikes against the jihadists in Syria.