More than 1,700 civilians have been killed by Russian air strikes in Syria since Moscow's air campaign began five months ago, a monitoring group said on Wednesday.

"Since September 30, Russian air strikes have killed 4,408 people including 1,733 civilians," the Britain-based Syrian Observatory for Human Rights said.

The civilian toll included 429 children and 250 women.

More than 60 percent of those killed were fighters, including 1,492 from rebel groups and Al-Qaeda's Syrian affiliate Al-Nusra Front, and 1,183 from the Islamic State group.

A ceasefire brokered by the United States and Russia came into effect on Saturday but does not include territory controlled by IS or Al-Nusra.

Russia, a key regime ally, has said it will continue striking "terrorist groups," but non-jihadist Syrian rebels say Moscow has continued bombarding other groups as well.

A US-led air coalition striking IS in Syria and Iraq since September 2014 has also continued its campaign.

Those strikes have killed 4,579 people in Syria including 374 civilians, among them 92 children, the Observatory said.

They have also killed 4,037 IS jihadists -- mostly foreigners -- and about 160 members of other hardline or jihadist groups, it said.

The Observatory relies on a broad network of sources on the ground to compile its reports.

It says it determines whether strikes were carried out by Syrian, Russian or US-led coalition aircraft based on the location of the raids, flight patterns and the types of planes involved.

More than 270,000 people have been killed since Syria's conflict erupted in March 2011, spiralling from widespread anti-government protests into a civil war.