Only 30 per cent of Russian air strikes in Syria target the Islamic State (IS) group while the rest are against opposition forces not affiliated with the jihadists, a senior US official said Wednesday.

"The Russian air strikes in Syria, primarily, are not attacking (IS)," Brett McGurk, US President Barack Obama's special envoy for the anti-IS coalition, told a news conference in Baghdad.

"If you run the numbers, it's about maybe 30 per cent are actually attacking (IS) and the rest of the air strikes are attacking other opposition groups... That are not affiliated" with the jihadists, McGurk said.

Russia began carrying out strikes in Syria on September 30 in support of its longstanding ally President Bashar al-Assad, whose regime is locked in a civil war with fractious opposition forces.

The conflict, which began in March 2011, has killed more than 250,000 people and displaced millions of