Members of the US-led coalition against Islamic State have called on Russia to cease air strikes they say are hitting the Syrian opposition and civilians.

In a joint statement on Friday, the US, UK, Turkey, Saudia Arabia said Russian strikes constituted an escalation and would "only fuel more extremism".

Russia, which eyewitnesses said launched fresh strikes on Friday, claims that it is targeting IS.

A senior Russian official says strikes could last for three to four months.

Alexei Pushkov, the head of the foreign affairs committee in Russia's parliament, added that the US had only "pretended" to bomb IS and promised that Russia's campaign would be much more effective.

The Russian Air Force began air strikes in Syria on Wednesday.

The strikes were Russia's first military engagement outside the borders of the former Soviet Union since the end of the Cold War.