Three months of Russian air strikes in Syria have killed more than 2,300 people, a third of them civilians, a monitoring group said on Wednesday.

Russia began conducting its air war in Syria on September 30 in support of embattled ally President Bashar al-Assad.

According to the Syrian Observatory for Human Rights, Moscow's strikes on Syria have killed 2,371 people so far. The toll includes 792 civilians, among them 180 children.

The raids killed 655 fighters from the Islamic State jihadist group, which Russia says it is targeting along with "other terrorist groups".

Another 924 opposition fighters -- ranging from US-backed rebels to members of Al-Qaeda's Syrian affiliate and IS rival Al-Nusra Front -- were also killed in the Russian strikes.

The Britain-based Observatory has an extensive network of sources inside Syria. Russia, a US-led coalition and the Syrian air force are all carrying out air raids in the country, but the Observatory differentiates between strikes based on the type of aircraft flown and the munitions used.

Russia has come under growing criticism from rebels, human rights groups, and the West for inflicting civilian casualties.

Amnesty International last week said Russian raids had killed hundreds of civilians, many in targeted strikes that could constitute war crimes.

US State Department spokesman Mark Toner echoed the accusations on Tuesday, saying Russian strikes have "killed hundreds of civilians, including first responders, (and) hit medical facilities, schools and markets."

More than 250,000 people have been killed since Syria's conflict erupted in March 2011 with anti-government protests.

Russia slams 'absurd' claims over civilian deaths in Syria
Moscow (AFP) Dec 30, 2015 - Russia on Wednesday slammed as "absurd" accusations by the United States that Russian air strikes in Syria are killing hundreds of civilians.

Russia has come under growing criticism from the West as well as human rights groups and Syrian rebels for inflicting civilian casualties in its two-month bombing campaign in the war-torn country.

The US State Department said Tuesday that the Russian strikes had killed hundreds of civilians" and hit "medical facilities, schools and markets".

"All of these anonymous and unsubstantiated statements about the alleged use of Russian aircraft on civilian targets in Syria is increasingly reminiscent of hypnotists' acts in travelling circuses," Russia's defence ministry said in a statement, calling the claims "absurd".

The Britain-based Syrian Observatory for Human Rights said Wednesday that Moscow's strikes had killed 2,371 people since they were launched September 30.

It said the toll includes 792 civilians, among them 180 children.

Russia's defence ministry said it deplored the "absolute silence" about the strikes by the United States, which is leading a coalition conducting its own bombing campaign against Islamic State jihadists.

It said the US raids had caused "mass casualties".

Amnesty International last week issued a damning report claiming that the Russian raids had killed hundreds of civilians, many in targeted strikes that could constitute war crimes.

Russia's defence ministry spokesman Igor Konashenkov had dismissed the report, saying it was made up of "cliches and fakes".

In the past two days, Russia's air force conducted 121 combat sorties and struck 424 targets in many parts of Syria.