Russian S-400 air defense missile systems travel along the runway at the Hemeimeem air base in Syria, about 50 kilometers (30 miles) south of the border with Turkey.

Russian troops are expanding a military base in central Syria, adding fortifications and developing its runways in a sign they intend to use it as their second air base in the country, Syrian activists said Thursday.

The work underway in the Shaayrat air base, about 40 kilometers (25 miles) southeast of the city of Homs, could also signal Moscow’s intention to step up airstrikes in the country’s central region where the Islamic State of Iraq and Syria (ISIS) group is active.

Russia has been conducting airstrikes in Syria since Sept. 30, using the Syrian government’s Basel al-Assad air base in the coastal province of Latakia. Su-24 bombers and helicopters take off on daily basis from that base, known by its old name of Hemeimeem, to bomb targets in areas in northern Syria.

A base in Homs province would facilitate Russian airstrikes in central Syria.

Bebars al-Talawi, a Homs-based activist, and Rami Abdurrahman, director of the Britain-based Syrian Observatory for Human Rights, both reported on Thursday major Russian activity in Shaayrat. However, they said there was no signs yet of any fixed-wing aircraft in the facility.

Al-Talawi said there were reports that Russian weapons, including artillery, have been brought into the base in recent days. He said runways at the base were being expanded, but so far only regime warplanes were using it.

Abdurrahman said work has been underway in Shaayrat for several weeks, with Russian troops working on building fortifications and working on the runways. In addition, Russian troops have begun using the T4 military air base, also in Homs province, for Russian helicopters conducting military operations around the ISIS-held towns of Palmyra and Qaraytan in the province.

There was no immediate comment from Russian officials.

A U.S. military official who spoke on condition of anonymity said Wednesday that the U.S. has seen Russian military personnel at Shaayrat, but not Russian aircraft.

At a separate Pentagon briefing Wednesday, Col. Steve Warren, a U.S. military spokesman in Baghdad, told reporters that the Russians have expanded out from the main base in Latakia and now have “facilities in about four, maybe five different spots in and around the Latakia area.”

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