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 US and its allies have insisted that President Assad should leave office, while Russia has backed its ally remaining in office.

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.

The latest developments 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.

The US defence official said: "A Russian official in Baghdad this morning informed US embassy personnel that Russian military aircraft would begin flying anti-Isil [IS] missions today over Syria. He further requested that US aircraft avoid Syrian airspace during these missions.

"We've seen media reporting that has suggested Russian missions have begun."

The official declared a US-led coalition would continue to fly anti-IS missions over Iraq and Syria as planned, and expressed criticism of Russia's move.

"While we would welcome a constructive role by Russia in this effort, today's demarche hardly seems indicative of that sort of role and will in no way alter our operations."