Russia appears to have begun carrying out air strikes in Syria against opponents of President Bashar al-Assad, a US defence official says.
The official said the strikes reportedly came in the area of the western city of Homs.
Washington was informed in advance that they were about to take place.
The development came amid reports that President Assad had formally requested Russian military support.
Reports from Russia say the upper house of the Russian parliament granted President Vladimir Putin approval to deploy the Russian air force in Syria.
Syria’s civil war has raged for four years, with armed groups – some of them Islamist, including the Islamic State (IS) group – have been fighting to overthrow the government.