Syrian government forces pounded rebels in Damascus overnight, battling to reverse opposition gains in the aftermath of the assassination of President Bashar al-Assad's security chiefs, Reuters said.

Army helicopters and tanks aimed rockets, machineguns and mortars at pockets of rebel fighters who have infiltrated the capital this week in an operation they call "Damascus Volcano".

Lightly-armed fighters have been moving through the streets on foot and attacking security installations and roadblocks.

But the heart of the city was quiet by 0400 GMT on Saturday, July 21 residents told Reuters.

The Syrian Observatory for Human Rights, an opposition group which monitors the violence in the country, said 240 people had been killed across Syria on Friday, including 43 soldiers.

The Observatory's combined death toll over the past 48 hours stands at 550, making it the bloodiest two days of the 16-month-old uprising against Assad.

On Wednesday a bomb killed four members of the president's narrow circle of kin and lieutenants, including his powerful brother-in-law, defense minister and intelligence chief.

"The regime has been rudderless for last three days. But the aerial and ground bombardment on Damascus and its suburbs shows that it has not lost the striking force and that it is re-grouping," opposition activist Moaz al-Jahhar said by phone from Damascus.

In the days since, rebels have pushed deep into the heart of the capital and seized control of other towns. On Thursday, they captured three border crossings with Iraq and Turkey, the first time they have held sway over Syria's frontiers.