Syrian government forces mounted heavy air strikes on Friday against rebel positions in and around the northern city of Aleppo, the focus of an insurgent offensive aimed at capturing areas controlled by President Bashar al-Assad.

The Syrian Observatory for Human Rights, a group monitoring the war, said fighting between the insurgents and government forces continued into the early hours of Friday, and Syrian army air strikes on rebel positions were continuous.

A Syrian military source said the attack had been repelled and heavy casualties had been inflicted on the insurgents, adding that the air force and artillery had been used to target the rebels who in turn had used heavy weapons in their attack.

The rebel attack spearheaded by Islamist groups was the most intense since insurgents took over most of the eastern part of Aleppo three years ago.

The Observatory's Rami Abdulrahman said the rebel forces had seized some buildings from government control on the northwestern city outskirts, but the advance was not of strategic importance.

He said town of Azaz in the countryside north of Aleppo had also been targeted in the air strikes.