BEIRUT (AFP) - At least 40 rebels were killed as a regime air strike on their base in the northern Syrian city of Aleppo triggered a huge explosion Thursday, a monitoring group said.

The Islamist rebels had shells stored at the base, which magnified the blast, the Syrian Observatory for Human Rights head, Rami Abdel Rahman, told AFP.

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

"At least 40 rebels from Islamist factions, including three leaders, were killed when Syria's army dropped a barrel bomb and fired a rocket on their base in Al-Shaar neighbourhood of Aleppo city," the Britain-based Observatory 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 criticised by human rights groups for its indiscriminate nature.