At least 20 civilians, including women and children, have been killed after Syrian government helicopters dropped explosive-packed barrel bombs on a district in Aleppo city, a monitoring group has said.

The Syrian Observatory for Human Rights said on Thursday that the attack hit the Muwasalat district of Shaar neighbourhood in the rebel-held city.

The Britain-based group had initially put the toll at 12.

A video of the aftermath of the strikes, posted online by the Observatory, showed ambulances arriving and emergency workers carrying stretchers rushing into a street shrouded in thick white smoke.

Women could be heard screaming hysterically at the scene of the attack.

Syria's military has increasingly resorted to using so-called barrel bombs, which rights groups have condemned as a particularly indiscriminate weapon that often kills civilians.

Barrel bombs are typically constructed from large oil drums, gas cylinders or water tanks filled with high explosives and scrap metal.

According to the Observatory and Syrian activists, the regime has stepped up its use of the crude weapons in recent weeks, as international attention focuses on the fight against the Islamic State of Iraq and the Levant group (ISIL).

'French bombmaker killed'

Meanwhile, overnight US air raids in Syria targeted and likely killed a French bombmaker who was a leader of an al-Qaeda offshoot accused of plotting attacks on the West, a US defence official has said.

David Drugeon, 24, was suspected of working with al-Qaeda veterans in what Washington calls the Khorasan group.

AFP news agency quoted an anonymous senior US defence official as saying " he was among the targets ... we think we got him".

The official added that it would take time to confirm the bombmaker's death with absolute certainly, as there were no US troops deployed on Syrian soil.

Source: Agencies