Syrian forces readied Sunday for a ground offensive against the rebel-held enclave of Eastern Ghouta outside Damascus, a monitor said, as residents of the capital braced for retaliatory shelling.

President Bashar al-Assad has in recent days been sending reinforcements from across the country to the edge of Eastern Ghouta, besieged by regime troops since 2013.

"The reinforcements are complete; the attack is just waiting for a green light," said Rami Abdel Rahman, the head of the Syrian Observatory for Human Rights.

An AFP correspondent in the Syrian capital said some residents were starting to pack bags and rent rooms outside of Damascus in anticipation of an operation now seen as imminent.

Parts of Damascus, which is under regime control, have been hit by rockets and mortars fired by rebels based in Eastern Ghouta.

Around 250 civilians were killed in a wave of government bombardment on the rebel-held region this month, and about 20 in retaliatory fire on Damascus itself.

Abdel Rahman said military reinforcements started arriving when the regime intensified its air strikes on Eastern Ghouta on February 5.

He said negotiations were taking place, with Russian involvement, for the evacuation of fighters whose footprint in Ghouta is limited but includes areas directly adjacent to Damascus.

The Syrian daily Al-Watan, which is close to the government, also reported on Sunday that negotiations were happening.

The government has already cleared swathes of territory around Damascus through local deals, whereby besieged anti-government forces were evacuated and bussed to other rebel-dominated areas.

If a deal is struck for Eastern Ghouta, rebels would likely be sent to the northwestern province of Idlib, which almost completely escapes government control.

But time appeared to be running out for an agreement Sunday and regime forces, who have been receiving aerial backing from Russia, looked on the brink of starting a major offensive.

"The collapse of the negotiations will signal the start of an assault," Abdel Rahman said.

An estimated 400,000 people still live in Eastern Ghouta, including hundreds in need of urgent medical evacuations.

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