London (AFP) - Only one in 20 Russian air strikes in Syria have targetted Islamic State (IS) fighters, Britain's Defence Secretary Michael Fallon said Saturday.

British intelligence services observed that five percent of the strikes had attacked the militant jihadist group, with most "killing civilians" and Free Syrian forces fighting against the regime of president Bashar al-Assad, Fallon told the Sun newspaper.

He said that Russia's intervention had further "complicated" the crisis, while suggesting that Britain should extend its own bombing campaign -- currently only operational against IS in Iraq -- to Syria.

"We're analysing where the strikes are going every morning," he told the paper. "The vast majority are not against IS at all.

"Our evidence indicates they are dropping unguided munitions in civilian areas, killing civilians, and they are dropping them against the Free Syrian forces fighting Assad.

"He's shoring up Assad and perpetuating the suffering."

Despite Russia's actions, Fallon insisted the British government would make the case to extend the RAF's strikes, saying it would be "morally wrong" not to target IS in Syria.

"We can't leave it to French and Australian, American aircraft to keep our own British streets safe," he added.

US President Barack Obama warned Friday that Russia's military engagement in Syria in support of strongman Bashar al-Assad was a "recipe for disaster".

Russia carried out a third day of air strikes in Syria Friday, saying it targeted Islamic State jihadists, as Putin faced increased international criticism over his military campaign.