t least two car bombs exploded in the centre of Syria's second city Aleppo on Wednesday, killing and wounding dozens of people, mainly soldiers, witnesses and a watchdog said.

The blasts occurred at Saadallah al-Jabiri Square near a military officers' club and a hotel, the Syrian Observatory for Human Rights said, adding without elaborating that there were "dozens of dead and wounded."

Syrian state television reported "three terrorist explosions" in the city and broadcast scenes of massive destruction of buildings in the square.

An AFP correspondent reported hearing two loud blasts which caused huge plumes of smoke.

The facade of the hotel was destroyed and a cafe collapsed. One person emerged from the hotel with his face covered in blood, the correspondent said. All government buildings were immediately closed after the blast.

Aleppo, Syria's commercial capital with a population of 1.7 million people, has been since mid-July one of the focal points of the conflict roiling the country.

Several districts of Aleppo were also bombed on Tuesday, the Observatory said, while pro-regime daily Al-Watan reported that extra troops were being sent to Aleppo.

"New reinforcements have arrived to support the army... and the armed men (rebels) are now fatigued and have begun to flee to their villages and towns in the province of Aleppo and elsewhere," it said in a report on Tuesday.

Fighting at the weekend rocked the centuries-old UNESCO-listed souk in the heart of Aleppo and sparked a fire that damaged hundreds of shops.

More than 31,000 people, have been killed in violence in Syria since the outbreak in March last year of an anti-regime revolt, which began as peaceful protests for reform but morphed into an armed insurgency when demonstrations were brutally crushed.

Most rebels, like the population, are Sunni in a country dominated by a minority Alawite regime.