At least 53 people, including children, have been killed in government air strikes in Syria's Aleppo city, activists have said.

Dozens of barrel bombs - oil drums or cylinders packed with explosives and shrapnel - were dropped by military helicopters on the heavily populated al-Qatriji neighbourhood, the Syrian Observatory for Human rights, a UK-based monitoring group, said on Sunday.

Activists told Al Jazeera that at least 53 people were killed in the air strikes, while the Observatory gave a death toll of 32, including three children.

Eight others were killed in shelling attacks by rebels on regime-controlled areas in the city, it added.

At least 40 air strikes hit the rebel-held areas on Sunday in some of the heaviest recent raids by Russian and Syrian government warplanes, the Syrian Civil Defence, also known as volunteer rescuing group White Helmets, have said.

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Rebels also hit government-held parts of Aleppo in what Syrian media said was an escalation in mortar attacks on the western parts of the country's largest city before the war.

State media said missiles fired on Hamadaniyah and the Midan areas by rebels left scores injured and several casualties in a second day of intense shelling of government-held areas.

According to the Observatory, at least 74 people in Aleppo have been killed in air strikes since May 31.

Zouhir al-Shimale, a local journalist, told Al Jazeera that Aleppo had been hit by intensive raids over the past few days.

"Every morning for the past few days over 50 air strikes and barrel bombs have targeted Aleppo. A local journalist was among those killed, while another journalist was injured.

"A rescue worker was also killed while he was saving an injured person. It happened within two minutes," Shimale said.

Government advance in Raqqa

In another development, the Syrian army crossed the boundary of Raqqa province after advancing in a major Russian-backed offensive against the Islamic State of Iraq and the Levant (ISIL) group, the Observatory said on Saturday.

Heavy Russian air strikes hit ISIL-held territory in eastern areas of Hama province, near the boundary of Raqqa, on Friday to facilitate the Syrian army's advance, the Observatory reported.

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The Observatory said at least 26 ISIL fighters and nine Syrian government and allied troops were killed in the fighting.

It was the first time that government troops had entered Raqqa province since they were ousted by ISIL fighters in August 2014.

The Syrian army was making its advances from the Athriya area of eastern Hama province, close to the provincial border with Raqqa.

The offensive brought troops to within less than 40km of Tabqa, which is the site of an airbase and a big reservoir, SOHR said.

ISIL, which controls large swaths of territory in Syria and Iraq, is fighting Syrian troops, US-backed fighters and other rebel groups in northern Syria and is facing an offensive by Iraqi government forces in their stronghold of Fallujah.

Russia's military intervention in support of Damascus in September has helped to bolster President Bashar al-Assad's government.