The Pentagon says Russia has in recent weeks built up a forward operating air base in Latakia in northwestern Syria, and sent at least 500 troops along with fighter jets, tanks, artillery and a slew of military hardware to the facility.

Russian military planes have conducted reconnaissance flights over Syria but not yet launched any airstrikes, a U.S. official said Friday.

The Pentagon says Russia has in recent weeks built up a forward operating air base in Latakia in northwestern Syria, and sent at least 500 troops along with fighter jets, tanks, artillery and a slew of military hardware to the facility.

“We have seen some ISR (Intelligence, Surveillance and Reconnaissance) activity from Russian ISR aircraft,” said Colonel Pat Ryder, the spokesman for the U.S. military’s Central Command, better known as CENTCOM, which is overseeing efforts against the Islamic State of Iraq and Syria (ISIS) group in Iraq and Syria.

“We are aware of Russian air activity but to my knowledge no strikes at this point,” he added.

Officially at least, the United States is unclear about Russia’s intentions in Syria, but observers expect Moscow to lend its support to the regime of President Bashar al-Assad, an old ally.

Ryder said the US military had not had any contact with Russian counterparts.

“The door is open to potential discussions in the future in terms of how Russia and the U.S. and the coalition might work together, but it would be premature for me to discuss what that might look like going forward,” he said.

The United States has said it would welcome Russian involvement in the coalition fight against ISIS militants, but warned against Moscow bolstering Assad’s regime by attacking moderate rebels on the ground.

“If the Russians were to take action against those groups instead of striking ISIL (ISIS), that’s something that we would definitely look upon with great concern,” Ryder said.