Syrian opposition rebels launched a wide-scale attack on regime strongholds late Thursday in the northern province of Aleppo, killing and wounding some 80 people, a monitoring group said.

"This is the heaviest attack by rebel forces Aleppo has witnessed since two years, as more than 400 shells fell on regime-held areas in Aleppo," Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, told dpa.

The attack came shortly after 13 Islamic brigades, mainly made of the al-Qaeda-affiliated al-Nusra Front, Mujahidi al-Islam, Ahrar al-Sharm and others announced that they have set "a joint military operation room" aimed at liberating Aleppo and its outskirts.

The heavy shelling targeted mainly regime-held areas like al-Zahraa, Khalidiyeh, Ashrafiyeh and al-Nile Street, killing four civilians and wounding more than 70, most of whom suffered serious wounds.

Coinciding with the heavy shelling, rebel fighters attacked from several fronts in the al-Zharaa neighbourhood, Chihan and al-Qalaa region in the old city of Aleppo.

In retaliation, Syrian regime planes bombarded the area of the clashes and rebel-held areas in Hritan, in the northern outskirts of Aleppo.