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.

Criticism

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."

Syria's civil war

What's the human cost?

More than 250,000 Syrians have been killed and a million injured in four-and-a-half years of armed conflict, which began with anti-government protests before escalating into a full-scale civil war.

And the survivors?

More than 11 million others have been forced from their homes, four million of them abroad, as forces loyal to President Assad and those opposed to his rule battle each other - as well as jihadist militants from IS and other groups. Growing numbers of refugees are going to Europe.

How has the world reacted?

Regional and world powers have also been drawn into the conflict. Iran and Russia, along with Lebanon's Hezbollah movement, are propping up the Alawite-led government. Turkey, Saudi Arabia and Qatar are backing the Sunni-dominated opposition, along with the US, UK and France.