What next for rebels, civilians in crumbling Ghouta?

As Russia-backed Syrian regime forces chip away at what remains of the last opposition bastion outside Damascus, experts say the future looks bleak for rebels there.

What is likely to happen to these opposition fighters, most of whom hail from the area, as well as tens of thousands of civilians still inside the besieged enclave?

How do things stand on the ground?

Government forces have retaken 80 percent of the former rebel bastion since launching a blistering air and ground assault to retake it on February 18, the Syrian Observatory for Human Rights says.

They have split the remaining areas under rebel control into three separate pockets, each held by different opposition factions, the Britain-based monitor says.

The rebel Jaish al-Islam group is isolated in a northern pocket around the main town of Douma, while hardliners from Ahrar al-Sham control a small area around the town of Harasta to the west.

In the south, Faylaq al-Rahman holds the town of Arbin and surrounding territory.

When fighting stops, negotiations are likely to determine the fate of opposition fighters and civilians there, analysts say.

"Fighting will stop at some point and they will negotiate," geography expert Fabrice Balanche said, adding that such talks could be prolonged.

As it seeks to win back control of the country, the Assad regime has often sought evacuation deals in rebel-held areas after brutal bombardment campaigns or suffocating sieges.

This was the case in the central city Homs in 2014 and in the northern city of Aleppo as regime forces retook full control of it in late 2016.

These so-called "reconciliation" deals saw rebels evacuated to other parts of the country under opposition control, including the northwestern province of Idlib.

Will Ghouta see an evacuation?

Eastern Ghouta's rebels have said they reject any evacuation, but on Friday announced they were prepared to hold direct United Nations-backed ceasefire talks with regime backer Russia.

Thomas Pierret, another Syria analyst, said the rebels stood little chance against the onslaught, which the Observatory says has claimed more than 1,400 civilian lives in a month.

However "excellent in urban warfare" Eastern Ghouta's rebels are, they "can't do much faced with the regime's fire power", he said.

When fighting stops, talks will decide what becomes of the rebels -- most of whom, Pierret said, originate from the region.

Aron Lund, a fellow with the Century Foundation, said that makes evacuations particularly devastating.

"The rebels in Ghouta are overwhelmingly from that area, which makes it especially painful and difficult to have fighters leave the area," he said.

"The Syrian government and the Russians are hoping to get some rebels to lay down their arms, while those who won't do so will be forced to evacuate to northern Syria," said Lund.

"Russia has offered to guarantee safe passage for surrendering fighters, who can bring their personal weapons and their families," he said.

The Observatory has said talks between local dignitaries or rebels on one side and the regime and its Russian ally on the other have yet to produce results.

The negotiations were on Jaish al-Islam possibly remaining in Douma and Russian military police being deployed there.

Those who refuse to put down their weapons could also head to rebel-held areas in the southern province of Daraa, the monitor said.

Faylaq al-Rahman rebels could head to territories held by the group in the northern province of Aleppo and neighbouring Idlib.

Balanche says Jaish al-Islam counts an estimated 6,000 fighters in its ranks, while Faylaq al-Rahman has some 3,000 fighters.

The Observatory says Ahrar al-Sham is several hundred fighters strong.

What fate for civilians?

Before the regime offensive started, the United Nations said 400,000 people lived in Eastern Ghouta, under a crippling government siege since 2013.

But as government forces advanced in recent days, 50,000 civilians have streamed out of the enclave into areas under regime control, according to the Observatory, despite their fears of their fate there.

When rebels surrendered in the past, Lund said, civilians who chose to stay were sometimes allowed to do so.

"Many people in Syria are just interested in surviving, saving their families, and getting on with their lives, whoever is in charge of their hometown," he said.

But "the question is how much of the Ghouta enclave's non-combatant population would be displaced in this way, and to what extent there can be a genuine choice about leaving or staying."

"Regime opponents and relatives of rebel fighters will fear revenge attacks, arrest and torture, and military-age men will fear being drafted to the army," he said.

"Their families may feel compelled to go with them."