It is the first Cameron's public statement on Vladimir Putin's intervention in the country, Censor.NET reports citing Ukrinform.
He claimed that Russian forces were not discriminating between Isis and non-Islamist opposition forces who are engaged in a bloody civil war against the Syrian regime.
"It's absolutely clear that Russia is not discriminating between Isil (Isis) and the legitimate Syrian opposition groups and, as a result, they are actually backing the butcher Assad and helping him and really making the situation worse," Cameron said.
The Prime Minister has been expected to extend British air strikes from Iraq into Syria, where some members of the US-led coalition are already active, for some time.
Russian bombing in Syria is only strengthening Islamic State - Obama
"Rightly, Russia and Assad have been condemned across the Arab world for what they have done and I think the Arab world is right about that. But we should be using this moment now to try to force forward a comprehensive plan to bring political transition in Syria because that is the answer for bringing peace to the region," Cameron added.
Michael Fallon, the Defence Secretary, suggested escalation was imminent in an interview with the Sun, where he said that the UK "can't leave it to French and Australian, American aircraft to keep our own British streets safe".