Syrian regime forces on Wednesday shelled two Damascus districts before troops backed by tanks swept through to carry out house-to-house raids in search of opposition fighters, killing at least 31 suspected rebels, activists said.

The violence is part of a dramatic surge in fighting over the past month in Damascus, which is just one of many fronts President Bashar Assad's regime is struggling to contain as the 17-month-old rebellion against his rule gains strength. Government forces are also engaged in a major battle for control of the northern city of Aleppo as well as smaller scale operations in the country's south, east and centre.

Around dawn Wednesday, regime forces in Damascus rained mortars down on the upscale Kafar Soussa neighbourhood and the adjacent Nahr Eishah area of the Syrian capital, activists said. Government troops appeared to be shelling the districts from Qasioun mountain overlooking the capital, a Damascus resident said on condition of anonymity for fear of retribution.

The attacks may have been designed to kill or capture rebel mortar teams who have used the two neighbourhoods in recent days to target the city's strategically located Mazzeh military airport, activists said.

The British-based Syrian Observatory for Human Rights said at least 20 people were killed in Kafar Soussa on Wednesday and that fierce battles were raging in an area just outside the neighbourhood between the rebels and government troops.

An activist in Kafar Soussa reached on Skype corroborated the observatory's report. He spoke on condition of anonymity because he feared reprisals.

Earlier, an activist who only wanted to be identified by the name Bassam for fear of retribution, said 11 people were killed in Kafar Soussa and that as many as 22 tanks stormed the district with about 20 soldiers on foot behind each one. He spoke via Skype from central Damascus.

House-to-house raids

Bassam and the observatory also reported heavy government shelling of Nahr Eishah early Wednesday. They said regime forces then conducted house-to-house raids in search of rebels. Bassam said as many as 12 people were killed in Nahr Eishah, while the observatory had no word on casualties.

It was not clear whether those killed in the two areas died in the shelling or later government raids. Activists, including the one reached by Skype in Kafar Soussa, spoke of execution-style killings in both areas.

The activists' reports could not be independently verified.

Syria's ongoing civil war has its roots in a mostly peaceful uprising against Assad's regime that began in March last year. The uprising grew increasingly violent as the government launched a brutal crackdown on protesters, prompting many to take up arms to forcefully overthrow Assad's regime.

The conflict has to date defied all international efforts to end it.