The death toll from Syrian air raids on rebel-held areas in the northern city of Aleppo has topped 40 over a 24-hour period, a monitoring group has said.

Barrel bombs killed 22 people in Hay Qataneh, Tariq al-Bab, Bustan al-Qasr, Bani Zeid, Mghayer and Lairamoun on Tuesdau, the Syrian Observatory for Human Rights said.

Another 21 people have been killed in another wave of barrel bomb strikes on the Mghayer district, it said.

Nine children figured among the dead, the Observatory added, warning that the toll could rise because "many people are in serious condition and there must bodies under the rubble".

US President Barack Obama has vowed to increase support for the rebels fighting Bashar al-Assad's Syrian regime and to help them confront their extremist rivals. 

"I will work with congress to ramp up support for those in the Syrian opposition who offer the best alternative to terrorists and a brutal dictator," Mr Obama said, in a speech to the West Point military academy.