Russia has begun carrying out air strikes in Syria against opponents of President Bashar al-Assad, Moscow has confirmed.
The strikes reportedly hit the rebel-controlled area of Hama province.
Washington was informed an hour before that they were about to take place.
Russian defence officials say the strikes targeted the the Islamic State group, but an unnamed US official told Reuters that so far they did not appear to be targeting IS-held territory.
Syria’s civil war has raged for four years, with an array of armed groups fighting to overthrow the government.
The US and its allies have insisted that President Assad should leave office, while Russia has backed its ally remaining in power.
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 permission to deploy the Russian air force in Syria.
The Russian defence ministry said the country’s air force had targeted military equipment, communication facilities, arms depots, ammunition and fuel supplies.
A Syrian opposition activist network, the Local Co-ordination Committees, said Russian warplanes hit five towns — Zafaraneh, Rastan, Talbiseh, Makarmia and Ghanto — resulting in the deaths of 36 people, including five children.
None of the areas targeted were controlled by IS, activists said.
A 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.»
Later, US state department spokesman John Kirby told reporters: «The US-led coalition will continue to fly missions over Iraq and Syria as planned and in support of our international mission to degrade and destroy Isil [IS].»