Syrian government forces and Islamist rebels engaged in fierce fighting in the contested northern province of Aleppo on Sunday, a monitoring group reported.

The clashes erupted after the rebels launched an attack on regime-held areas in the southern parts of Aleppo, the Syrian Observatory for Human Rights added.

Airstrikes by unidentified jets hit rebel-held districts in the city of Aleppo, the capital of the province with the same name, the Observatory said. No casualties were reported.

Shells fired by rebels, meanwhile, hit regime-controlled neighbourhoods in suburbs of Aleppo, leaving at least three civilians dead, said the Britain-based watchdog that relies on a network of activists inside Syria.

Syria's state news agency SANA reported that chief of army staff Ali Ayoub had inspected government troops in Aleppo where he was posted on the "nature of their combat missions" there.

The agency did not directly mention Sunday's reported clashes in Aleppo.

Earlier on Sunday, a spokesman for Jaish al-Fatah, a coalition of Islamist rebels, said in an online video that the grouping planned a "major battle" to break the regime siege on the Aleppo city.

The video could not be independently verified.

Forces of Syrian President Bashar al-Assad are imposing a tight blockade on the rebel-held eastern section of Aleppo after they blocked all supply routes into the area earlier this week.

On Thursday, the Syrian government said it had opened three humanitarian corridors in Aleppo to help an estimated 250,000 to 300,000 civilians leave the opposition-held neighbourhoods.

State media said that dozens of families have departed through corridors to regime-controlled areas - a claim that rebels denied.

Once Syria's commercial hub, Aleppo has been divided between government forces in the west and rebels in the east since fighting erupted for control of the city in mid-2012. 

Syria's conflict started with peaceful pro-democracy protests in March 2011.

It soon developed into a full-blown war that has devastated the country. Hospitals and medical volunteers have been major casualties in the strife.

On Sunday, a series of airstrikes on a rebel-held area in southern Syria killed at least five civilians and put a hospital out of service, according to the Observatory.

The bombardment by unidentified warplanes targeted the opposition-controlled town of Jasem in the southern Daraa province.

One air raid hit the town's hospital, killing a pharmacist and leaving an unspecified number of people injured, the Observatory reported.

The attack is the second this week against a medical facility in rebel-held areas in war-torn Syria.

On Friday, a maternity hospital, supported by the aid group Save the Children, was hit in an air bombing of an opposition-held area in the north-western Idlib province.

That strike caused unspecified casualties and heavily damaged the hospital.

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