At least 40 rebels were killed as a Syrian regime air strike on their base in the northern Syrian city of Aleppo triggered a huge explosion Thursday, France24 cited a monitoring group as saying.

The militants had shells stored at the base, which magnified the blast, said the Syrian Observatory for Human Rights head Rami Abdel Rahman.

"The shells exploded when the strike happened, and witnesses there described an explosion like an earthquake," Abdel Rahman said.

Al-Shaar falls within opposition-controlled east Aleppo city, which is regularly bombarded by regime warplanes and helicopters.

Abdel Rahman said the fighters targeted were part of the Northern Front, a coalition of Islamist factions formed in December.

The regime's use of barrel bombs -- old containers full of explosives -- has been criticized by human rights groups for its indiscriminate nature.