U.S.-led air strikes in Syria have killed more than 1,000 jihadists in the past three months, nearly all of them from the Islamic State group, a monitoring group said Tuesday.

"At least 1,171 have been killed in the Arab and international air strikes (since Sept. 23), including 1,119 jihadists of the Islamic State group and Al-Nusra Front," said the Syrian Observatory for Human Rights, which relies on a network of activists and medics across the war-ravaged country for its information.

Among the dead were 1,046 members of ISIS, which has seized large chunks of Iraq and Syria and is the main target of the air campaign.

Seventy-two of those killed were members of Al-Qaeda's branch in Syria, the Al-Nusra Front, while another was a jihadist prisoner whose affiliation was unknown, an Observatory statement said.

The remaining 52 were civilians.

ISIS has declared a "caliphate" in the parts of Iraq and Syria that it has overrun, and its militants have been accused of widespread atrocities, including beheading Western hostages.

On another front, the Observatory reported the deaths of 37 civilians in regime air raids across Syria on Tuesday, among them nine children in ISIS stronghold Raqa alone.

Syria's war began as a peaceful pro-democracy revolt. It later morphed into a brutal civil war after the regime unleashed a massive crackdown against dissent.

Thousands of people, most of them civilians, have been killed in air strikes since July 2012 when the regime's air force was first deployed in the war.