Damascus is seeing its heaviest fighting between rebels and government forces since the conflict began at the beginning of last year, a monitoring group said late Sunday.

Gunfire was heard Sunday in several neighborhoods of the Syrian capital, and protesters came out onto the streets and blocked roads to protest government offensives in those districts, the London-based opposition Syrian Observatory for Human Rights said.

It said ambulances were sent in to transport injured government forces away and killings were also reported among citizens but the numbers of the casualties were not immediately possible to verify, given the intensity of the fighting.

"These are the most intense clashes Damascus has ever seen since the beginning of the uprising" in March 2011, the group said.

The opposition Local Coordination Committees of Syria said the government shelled the neighborhood of Tadamon in south-eastern Damascus, resulting in casualties among residents in their homes. The government sent in snipers to the area as residents fled, it said.

The Syrian Observatory for Human Rights identified Tadamon as one of the neighbourhoods where the heaviest clashes were taking place.