Twenty-five children have reportedly been killed in airstrikes on heavily crowded areas in a town in eastern Syria that was controlled by the Islamic State group, the United Nations Children's Emergency Fund (UNICEF) said on Monday.

UNICEF quoted its local partners from Syria as saying that 25 bodies of children were pulled out from under the rubble by health workers in the al-Quria town of Deir al-Zor province, according to Reuters.

"Three attacks reportedly hit heavily crowded areas including a mosque during prayer time... UNICEF deplores these attacks and calls on all parties to the conflict to keep children out of harm's way," UNICEF was quoted by the agency as saying.

ABC Online quoted the Syrian Observatory for Human Rights monitoring group as saying that around 50 people, including 30 civilians, were killed in the attacks. According to the group, Syrian or Russian warplanes carried out the strikes, killing dozens of people.

The UNICEF was quoted by ABC Online as saying nothing could justify the killings of children. "No matter where they are, nor under whose control they live," UNICEF said in a statement.

The province of Deir al-Zor links the Syrian city of Raqqa, which also happens to be the de facto capital of Isis, to the other territories that the group controls in Iraq, according to the Reuters report.

Russian and Syrian warplanes also overnight carried out airstrikes in ISIS-controlled areas to wipe out the last remaining supply group of the militants in Aleppo, Syria's largest city, ABC Online reported.

Meanwhile, the opposition in Syria has called on UN Secretary-General Ban Ki-moon to investigate Russia's alleged usage of air-delivered incendiary weapons and cluster bombs in the country.