BEIRUT: Syrian rebels fired dozens of mortar bombs into central Damascus on Monday, hitting a high-security area within a kilometre (less than a mile) of President Bashar al-Assad's residence, residents and a security source said.

The military retaliated with artillery fire from Mount Qasioun, overlooking the Syrian capital. “I've heard dozens of regime shells so far, pounding rebels,” one resident said.

It was some of the heaviest fighting in the heart of the city since an uprising against Assad erupted two years ago.

The security source, who asked not to be identified, said mortar rounds had landed in Ummayad Square, a major intersection where military headquarters and state television are located.

On Sunday, mortar bombs hit the car park of the state television building, residents said. One resident said the latest rebel bombardment began at 6:30 a.m. (0430 GMT) and was still continuing. “I don't know what is going on exactly other than that the city is under attack,” she said, adding that she could hear sirens from the streets.

State television reported dozens of casualties at the Opera house, across the street from the headquarters of Assad's Baath party the Air Force Intelligence building.

The mortar bombs appeared to be coming from rebels who had pushed into the Kfar Souseh district, a few hundred metres (yards) from Ummayad Square, but there were no immediate reports of the insurgents trying to advance further.

Assad's forces have retained control of central Damascus and most other Syrian cities, while losing swathes of territory elsewhere, especially in the north and east.

The conflict, in which the United Nations says at least 70,000 people have been killed, has already forced more than a million Syrians to flee the country. Sustained fighting in Damascus could send thousands more refugees into neighbouring states, especially Lebanon, which already hosts 370,000 of them.