A car bomb killed at least seven civilians on Tuesday when it exploded in a southern Damascus neighborhood, a monitoring group said.

The Syrian Observatory for Human Rights said 15 people had also been injured in the blast, which hit the district of Tadamon, where President Bashar Assad's forces have been fighting rebels for months.

Syrian state television said "terrorists" were behind the explosion, a term it commonly uses to refer to rebels who are fighting to topple Assad.

The Britain-based Observatory, which has a network of activists across Syria, said the blast had destroyed several buildings.  

Meanwhile, Syrian insurgents pressed a new assault in the strategic, heavily-defended southwestern suburbs of Damascus on Tuesday, saying they aimed to break an army blockade of rebel-held areas.

Rebels fired mortars at government forces while army shelling hit the suburb of Derayya, the scene of recurrent fighting during the 30-month-old Syrian conflict, activists and rebels said.

Insurgents have dug in around a crescent of suburbs outside the capital for about a year and a half, but Assad's forces have halted or reversed their advances in several strategic districts that are now effectively blockaded.

Assad's air power and better-equipped ground forces have so far fended off attacks by rebels whose main arms supply routes into the suburbs were severed by the army earlier this year.

Fighting in Damascus subsided when the United States threatened punitive military action against Assad, but picked up after a diplomatic bargain for Syria to give up its chemical weapons averted the immediate threat of a strike.

The rebels' new offensive, which they named "If You Return, We Return," united several factions operating in the area, including the Al-Qaida-linked Nusra Front.

Ahrar al-Sham, another big Islamist faction, also said it was playing a major role, along with the Sahaba Battalions, a powerful Damascus-based rebel unit.

A Sahaba commander said the operation aimed to relieve residents of the Derayya and Mouadamiya areas in the capital's southwestern outskirts from a "suffocating blockade" that was cutting off food, medicine and arms.

"There are big shortages of humanitarian and medical supplies," said the spokesman, who goes by the name Abu Moaz, adding that the operation had started on Monday night.

"The situation is very bad. No food, no water. There had to be military action," he said.

The southern suburbs will be a tough target for the rebels, given that they house several major military sites, including a military airport and a base for elite Republican Guard forces.

A Damascus-based activist who calls himself Bassem Mohammed said the rebels were unlikely to gain from the latest assault.

"The regime's top forces are there," Mohammed said. "We on the ground consider these to be 'show' operations from the Free Army, especially in cooperation with Ahrar al-Sham and Nusra. It's a war of attrition. There is no winner and no loser."

The British-based Syrian Observatory for Human Rights, which has a network of sources across Syria, said rebels had taken control of the Tel Rakees and Tel Bouza areas south of the capital and had destroyed two vehicles.

A video posted online purported to show rebel fighters firing mortars at government positions.

More than 100,000 people have died in Syria's conflict, which began in March 2011 with peaceful protests against four decades of Assad family rule and later descended into civil war.

Mouadamiya was one of the sites struck by a chemical attack last month that killed hundreds of people and which Western powers blamed on Assad's forces. The government denied this.