Russia is expanding an air base in Homs Province in western Syria to accommodate its fighter jets, the Syrian Observatory for Human Rights has said.
The monitoring group said on December 3 that the Russians were adding fortifications and developing runways at the Shayrat air base, about 40 kilometers southeast of Homs city, in a sign they intend to use it as their second air base in the country.
The work under way could signal Moscow's intention to step up air strikes in the country's central region where the Islamic State group is active, it said.
A U.S. official said the United States believes Russia had been moving equipment to the Shayrat base and some of the equipment is already operational.
Russia has been conducting air strikes in Syria since September 30, using the Syrian government's Basel al-Assad air base in the coastal province of Latakia. Su-24 bombers and helicopters take off daily from that base.