Syrian warplanes hit a popular market in a rebel-held district of Aleppo on Thursday killing at least 33 civilians, the Syrian Observatory for Human Rights said.

Several people were also wounded in the attack on the market in the Halak neighbourhood of northeastern Aleppo, the Britain-based monitoring group said.

It also reported at least 21 people killed in clashes between the jihadist Islamic State of Iraq and the Levant (ISIL) and other rebel groups including the Al-Qaeda-affiliated Al-Nusra Front in the eastern province of Deir Ezzor.

Rebel-held areas of Aleppo have come under massive air assault since mid-December as government forces try to regain full control of Syria's second city and one-time commercial hub.

"A Syrian fighter jet fired a missile at the (Halak) neighbourhood, and within minutes it fired another," Observatory head Rami Abdel Rahman told AFP.

The Aleppo Media Centre run by local activists said that "two residential buildings were destroyed and several shops set ablaze".

Activists posted a YouTube video showing an ambulance trying to make its way across rubble to the site and the facades of buildings badly damaged.

Youths are seen carrying the body of a man in a rug as others dig through rubble for victims.

A man is heard crying "Allahu akbar" (God is the greatest) as he picks up body parts and wraps them in clothing.

The authenticity of the video could not be immediately verified.

- School hit -

Thursday's air raids come a day after the Observatory reported that twin air strikes on a school in the Ansari neighbourhood controlled by insurgents killed 18 people, 10 of them children.

Aleppo-based citizen journalist Mohammed al-Khatib told AFP by Skype that the children were "holding a drawing exhibition when two air strikes, 10 minutes apart, struck the school".

The Observatory, which relies on activists and medics on the ground for its reports on the three-year war in Syria, said at least one teacher was also among the dead in Wednesday's strikes.

Rebel bombardments of regime-held areas of Aleppo have also intensified in past weeks.

On Sunday, rebels fired mortar rounds into parts of the Old City where government forces are entrenched and into other areas of Aleppo, killing more than 20 people.

The rebels have been trying to advance into the Old City.

Regime forces have also stepped up the use of barrel bombs dropped by helicopter on rebel positions, according to the Observatory.

In eastern Syria, ISIL forces launched an offensive aiming to retake areas of Deir Ezzor province near the border with Iraq that they lost in February to groups including Al-Nusra front.

Abdel Rahman said at least 21 people were killed in Thursday's fighting.

ISIL already controls large swathes of the neighbouring province of Raqa.

In early January, many rebel groups angered by abuses attributed to ISIL and by its desire for dominance attacked the jihadists in rebel-held areas of northern Syria.

Since then, the Observatory estimates that the inter-rebel fighting has caused thousands of deaths.