MOSCOW (Reuters) - The Russian air force is using KH-29L air-to-surface laser-guided missiles in Syria, the RIA news agency reported on Sunday, citing Colonel Igor Klimov, an air force spokesman.

The high-precision missiles are being used by Su-34 and Su-24 aircraft, Klimov said.

On Saturday, Russia said it would step up its air strikes in Syria, escalating a military intervention which Moscow says is weakening Islamic State militants, but which Western powers say aims to support President Bashar al-Assad.

The British-based Syrian Observatory for Human Rights has said at least 39 civilians have been killed since the start of the Russian air strikes on Wednesday. It said 14 fighters, mostly Islamic State militants, had also been killed.