BEIRUT - 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.

On Sunday dozens of rockets and artillery slammed into three key towns in Eastern Ghouta, he told AFP.

The last rebel bastion near Damascus, Eastern Ghouta is held by two main Islamist factions, Jaish al-Islam and Faylaq al-Rahman, although jihadists also have a foothold there.

The regime is keen to regain control of the area to halt the deadly salvo of rockets and mortars that those factions have fired on the capital.

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

One of them said he was planning to temporarily move his parents out of their home in the capital's east, which is regularly hit in rebel shelling.

"I'd rather drop my parents off in our village in Wadih al-Qalaa," in the government-controlled province of Latakia, said 29-year-old Karim. "They'll be safer there, and I can bring them back as soon as it's calm in their neighbourhood," he told AFP.

According to the Observatory, the regime began dispatching military reinforcements to Eastern Ghouta on February 5, the same day it launched a fierce five-day bombing campaign on the region. Air strikes have left around 250 civilians dead, with retaliatory rocket fire on Damascus killing about 20, the monitor said. Abdel Rahman said negotiations with Russian involvement were taking place for the evacuation of jihadist faction Hayat Tahrir al-Sham (HTS), 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 the talks on Sunday.

But rebels have denied the claim, with top Jaish al-Islam figure Mohammad Alloush telling AFP there were no negotiations.

"We reserve our legitimate right to defend ourselves. We opened the door for a political solution and participated in negotiations to stop the bloodshed in Syria, but the other side breached these agreements and broke every ceasefire," he said.

Faylaq al-Rahman spokesman Wael Alwan also told AFP his group had "no correspondence or negotiations with the criminal regime or its allies."

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, HTS jihadists would likely be sent to the northwestern province of Idlib, which is almost completely out of 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 Observatory said more than 240 rockets had rained down on Eastern Ghouta in the space of an hour on Sunday evening.

"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.