US officials say that Russia appears to have begun carrying out air strikes in Syria against opponents of President Bashar al-Assad.

They say the strikes reportedly came in the area of the city of Homs, and that 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. Syria's civil war has raged for four years.

Armed groups - some of them, including the Islamic State group, Islamist - have been fighting to overthrow the government.