Syrians make their way through debris following a reported air strike in the rebel-held town of Beit Sawa, in the Eastern Ghouta region on the outskirts of Damascus

Three weeks of ferocious bombardment of Syria's Eastern Ghouta have crushed illusions of a lasting truce in the rebel enclave which Damascus appears determined to recapture at all costs.

A blistering air campaign has reduced a "de-escalation" deal in Eastern Ghouta to tatters, just as regime and opposition representatives struggle to progress at sputtering peace talks in Geneva.

At least 193 civilians, including 44 children, have been killed in three weeks of Syrian government raids and artillery fire on the area, according to the Britain-based Syrian Observatory for Human Rights.

Rebels based in the area have also ramped up rocket fire onto Damascus, killing 29 civilians and wounding more than 200 in the capital.

Eastern Ghouta, the last opposition stronghold near Syria's capital, was one of four "de-escalation zones" agreed in May by powerbrokers Russia, Iran and Turkey.

The deal aimed to pave the way for a nationwide ceasefire and political settlement that would finally put an end to the conflict that has killed 340,000 since 2011.

A reduction in violence has more or less held in the other three zones in Syria's northwest, centre and south.

In mid-November, a relative calm that had prevailed over the Eastern Ghouta area ended with an explosive bombing campaign by regime forces.

"Eastern Ghouta is suffering the fiercest bombing and worst situation since the war began," said Thomas Pierret, a Syria specialist at the University of Edinburgh.

"We're really very far away from a reduction in violence," said Pierret, adding that the de-escalation zone had been reduced to "a big joke".

- 'Half my house standing' -

Eastern Ghouta is home to an estimated 400,000 people struggling to survive under a devastating four-year government siege that has made food and medicine unavailable or too expensive.

The United Nations has warned that 500 of them, mainly women and children, are at risk of dying without immediate medical care.

On November 28, Damascus nominally accepted a new Russian-proposed truce in Eastern Ghouta, to coincide with the resumption of UN-led peace talks in Geneva.

A Syrian man carries the body of a child who was killed in a reported air strike in the rebel-controlled town of Hamouria, in the Eastern Ghouta region on the outskirts of Damascus

But it has since ratcheted up its bombing.

Maysoun, a nurse living in the large Eastern Ghouta town of Douma, says she can vividly remember the day air strikes picked up again.

"I came back from the market to find half my house was left standing," said the 30-year-old, who has since been staying with a friend.

"I hadn't expected that much destruction. Usually, it's just the doors and windows that are affected," she said.

Rajaa, a mother of four now pregnant with her fifth child, lives in Hammuriyeh -- one of the most heavily-bombed towns in recent weeks.

"When they hear the planes, the children cry and don't want to stay at home anymore. They're traumatised," she told AFP.

- 'On the capital's doorstep' -

President Bashar al-Assad has repeatedly pledged his forces would retake all of Syria, and observers point to several reasons he appears to have doubled down on Eastern Ghouta now.

French cartographer and analyst Fabrice Balanche said the de-escalation deal had served as a "temporary" arrangement to allow the regime to redeploy troops elsewhere.

With key battlefronts against rebels frozen, Russian-backed government forces could focus on ousting the Islamic State group from Syria's centre and east, Balanche said.

Now, with IS territory reduced to a tiny sliver, Assad could comfortably resume the push for Eastern Ghouta.

"The regime needs to get rid of this pocket," used by opposition factions to lob rockets on Damascus and launch offensives on nearby troops, Balanche said.

"There's no question for the regime about allowing rebels to constitute little republics in these territories," he said.

For Sam Heller, an expert at The Century Foundation, Eastern Ghouta's location "on the doorstep of the capital" has kept it a priority for Assad.

The Syrian government has worked at "digesting it over time, through a combination of siege (and) periodic military pressure", said Heller.

- Other zones next? -

The other three de-escalation areas -- the south, Homs in central Syria, and northwest Idlib -- are lower on Assad's list and have so far remained relatively quiet.

"The zones' success rate ranges from 20 percent in Eastern Ghouta to 80 percent in the other regions," said Observatory head Rami Abdel Rahman.

The southern zone borders Israel, making a government push sensitive. The "status quo" would likely prevail there, according to Balanche.

Rebel enclaves in Homs province are smaller and less threatening -- but Idlib presents a major challenge, with most of the province held by jihadists once loyal to Al-Qaeda.

Tens of thousands of rebels who were bussed out of opposition zones under local truce deals were dumped in Idlib, along with thousands of civilians.

An assault to take it would take much preparation, said Balanche. "There are many rebels, battle-hardened men ready for suicide."