Syrian rebels stormed a government-held area in northeastern Damascus for the second time in three days, sources on both sides said, pressing the boldest assault on the capital by opposition fighters in several years.

The spokesman for one of the main rebel groups involved in the attack said the new offensive began at 0300 GMT, targeting an area rebel fighters had seized from government control on Sunday before being forced to retreat.

"We launched the new offensive and we restored all the points we withdrew from on Monday. We have fire control over the Abassiyin garages and began storming it," Wael Alwan, spokesman of rebel group Failaq al-Rahman, told Reuters.

Failaq al-Rahman rebels are fighting alongside fighters from the Free Syrian Army (FSA) and a hardline al-Qaeda-linked alliance called Tahrir al-Sham.

A Syrian military source said that rebel fighters had entered the area, setting off a car bomb at the start of the attack. The source said a group of rebels that had entered the area had been encircled and were "being dealt with".

The rebel groups have launched the assault from their Eastern Ghouta stronghold to the east of the capital.

Government forces have escalated military operations against Eastern Ghouta in recent weeks, seeking to tighten a siege on the area. The rebel assault aims partly to relieve that pressure.

The fighting has focused around the Abassiyin area of the northeastern Jobar district, some 2km east of the Old City walls, at a major road junction leading into the capital.

A monitor reported a large explosion at dawn followed by fierce clashes, shelling and government air strikes on opposition positions.

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"There was a big blast at dawn, most likely due to a car bomb attack by the rebels against a regime position between the districts of Jobar and Qaboun," Rami Abdel Rahman, head of the UK-based Syrian Observatory for Human Rights (SOHR), told the AFP news agency.

The Syrian state news agency SANA said that 12 people were wounded in rebel shelling in the second wave of the rebel offensive.

"The Syrian army is facing attempts to advance by terrorist groups ... north of Jobar and is surrounding them," SANA reported.

In the first wave, rebel groups scored gains in Jobar, even briefly advancing into Abbasid Square close to the capital's Old City.

'Rebels were pushed back'

However, forces loyal to President Bashar al-Assad drove them back by nightfall and began a fierce bombing campaign on Monday, the Observatory said.

Al Jazeera's Imtiaz Tyab, reporting from neighbouring Lebanon, said the offensive was seen as an achievement by the rebel fighters, who suffered a string of losses against Assad and his allies for the last 18 months.

"Despite these losses against the network of groups led by Assad, it would appear they still have the capabilities to hit back," he said.

Clashes on Sunday and Monday killed at least 72 people, including 38 government soldiers and 34 rebels, according to the Observatory.

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A rebel commander said the Syrian army was intensifying its shelling on areas they had advanced in Jobar and towns across Eastern Ghouta.

"The bombardment is on all fronts ... there is no place that has not been hit ... the regime has burnt the area by planes and missiles," said Abu Abdo a field commander from Failaq al-Rahman brigade.

The Syrian government appears to be employing the same strategy it has used to force effective surrender deals on rebels elsewhere around the capital through escalated bombardment and siege tactics.

Rebel fighters have been granted safe passage to insurgent-held areas of northern Syria under such agreements.

The SOHR said at least 143 air raids were conducted by the Syrian army on rebel held eastern parts of Damascus, mostly targeting Jobar, since the rebels launched their offensive.

Syria's conflict erupted in March 2011 with protests against Assad's rule but has evolved over the years into a complex civil war.

Nearly half a million people have been killed and half of the country's pre-war population has been displaced.

Repeated peace talks over the years have failed to bring about a political solution, but another round of negotiations is due to begin in Geneva on Thursday.