BEIRUT: The Syrian Observatory for Human Rights said on Thursday that Russia was expanding an air base in Homs province in western Syria to accommodate its fighter jets, and was already using another, further east, for helicopter sorties.

The first, the Shayrat air base, is southeast of Homs city, and the second, at al Tayas towards the Islamic State-controlled city of Palmyra, were likely to be used to strike the group in the area, said the Britain-based Observatory, which monitors the war through contacts on the ground.

A U.S. official said the United States believed Russia had been moving equipment to the Shayrat base. The official could not confirm what specific types of equipment were being moved into the air base, but said the United States believes some of the equipment is already operational.

Russia's defence ministry did not immediately respond to questions from Reuters about the two sites.

Russia began air strikes against Syrian insurgents in support of President Bashar al-Assad on Sept. 30, mainly from bases further west including in coastal Latakia province.

The air campaign was launched ostensibly to target Islamic State, but the United States, which is waging its own separate bombing runs against the group in Syria and Iraq, says Russian warplanes have mostly hit other, foreign-backed rebels.

The White House said on Monday, however, that it has seen "some intensification" of Russian air strikes against Islamic State during the past few weeks.