At least 82 people including 58 civilians were killed in Russian and regime air strikes on an Islamic State group-held area of eastern Syria, a monitor said Sunday in a new toll.

"Three Russian and Syrian regime air raids on the region of Al-Quriyah, southeast of Deir Ezzor city, killed 58 civilians," the Syrian Observatory for Human Rights said.

It added that 24 other people were killed, without specifying wether they were civilians or IS fighters.

The Britain-based Observatory, which has a network of sources in Syria, initially reported that 47 people including 31 civilians died in the raids around Al-Quriyah.

The UN children's agency UNICEF said 25 childre were reportedly among those killed in Saturday's raids on Al-Quriyah.

"Three attacks reportedly hit heavily crowded areas including a mosque during prayer time," it said. "Health workers were reported to have pulled bodies of children out from under the rubble."

Russian warplanes have been carrying out an air war in support of President Bashar al-Assad since September 2015.

IS holds around 60 percent of Deir Ezzor city, the capital of the province of the same name, which is next to the jihadist-held Raqa province.

More than 280,000 people have been killed since Syria's conflict erupted in March 2011, after a widespread protest movement evolved into a complex, multi-front war that has drawn in global powers.