BEIRUT (Reuters) - Air strikes by Russian warplanes on the Islamic State-held Syrian city of Raqqa killed 42 people earlier this week, including 27 civilians, monitoring group the Syrian Observatory for Human Rights said on Friday.

Fifteen Islamic State fighters made up the remainder of the death toll, the Britain-based Observatory said, after a series of strikes on Tuesday that hit the group's Syrian stronghold.

The bombings add to a civilian death toll from Russian strikes, which the Syrian Network for Human Rights, another monitoring group, said on Monday stood at 254 people killed in just over a month.

It was also a rare heavy Russian bombardment targeting Islamic State rather than other insurgent groups.

Russia's air force intervened in Syria's four-year civil war on Sept. 30 on the side of President Bashar al-Assad, launching a campaign it said targeted Islamic State.

The United States says Russian raids have mostly hit other Syrian insurgents, including foreign-backed and more moderate groups.

The Russian intervention has marked a new phase in the war, intensifying fighting in the country's west and northwest between rebels and pro-government forces backed also by Iranian troops and Lebanese Hezbollah fighters.

Russian strikes have hit Islamic State-held areas further east, but have been mostly concentrated in the west.

Its air force said on Tuesday it had flown 1,631 sorties and struck 2,084 militant targets since the start of the campaign.

More than 120,000 people have been driven from their homes since Moscow's bombing campaign started, the United Nations says. The war has killed an estimated 250,000 people and driven more than 11 million from their homes.