At least 53 people, including children, have been killed in regime 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 neighborhood, 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 strikers, while the Observatory gave a death toll of 32, including three children.

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

Opposition forces also hit regime-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.

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 has been hit by intensive raids over the last 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.

PHOTO CAPTION

Men inspect damage after an airstrike on Aleppo''s opposition held al-Hallak neighborhood [Reuters]

Al-Jazeera