Syrian warplanes have battered the enclave's towns since Monday, trapping thousands of families in makeshift bomb shelters and overwhelming rescue workers.

World powers failed to back an appeal by UN officials for a month-long ceasefire to allow for desperately needed aid deliveries and medical evacuations.

Bombing raids resumed on several Eastern Ghouta towns on Friday morning, leaving at least six civilians dead, according to the Syrian Observatory for Human Rights, a British-based war monitor.

Residents had taken advantage of a lull in bombing on Friday morning to begin clearing rubble from their doorsteps and sweeping away broken glass.

In Douma, some civilians were seen scouring for salvageable items, while others rushed to the market to find food and other supplies.

But around mid-morning, an announcement blasted over mosque minarets warned of incoming strikes: "Surveillance plane in the sky. Clear the streets."

Soon after, bombing raids began and left four people dead in the town.

One raid hit Douma's local civilian council, starting a fire on the upper levels.

Multiple strikes also killed two civilians in Erbin, where medics reported being overwhelmed by the relentless bombardment.

"From 2011 until now, there has never been the level of bombardment we've seen in the last 96 hours," said Hamza, one of doctors treating the wounded, on Thursday.

- 'Humanitarian disaster' -

The Syrian Observatory for Human Rights said at least 75 people died on Thursday, three of wounds suffered the previous day.

That brought the civilian death toll since Monday to more than 230.

Eastern Ghouta is home to an estimated 400,000 people who have lived under crippling government siege since 2013.

More than 4,000 families live in basements and bunkers for fear of air strikes, according to Save the Children.

CARE International said the intensity of the air strikes had made it extremely difficult for relief workers to assist the needy.

"Our partners are having a hard time moving around, so how can they reach vulnerable people?" the group's communications director for Syria, Joelle Bassoul, asked.

"If there is no ceasefire, if this is all left unheard, we cannot imagine the scale of the humanitarian disaster," she said.

Eastern Ghouta is supposed to be one of four "de-escalation zones" declared last year in a bid to reduce the bloodshed.

But as the last rebel-held district on the capital's doorstep, the government is more determined than ever to clear it, said Nick Heras, an analyst at the Center for New American Security.

"This strategy of laying siege to the Ghouta is at its basic element a strategy of collective punishment, of making the entire population of the area suffer for its choice to rebel against Bashar al-Assad and his regime," he told AFP.

Sensing the mounting international pressure for a fighting freeze in Ghouta, Assad is looking to quickly "make the collapse of the opposition in the Ghouta a fact on the ground that cannot be reversed by anybody."

- Russia, US cross swords -

UN aid officials appealed for a month-long humanitarian truce to allow aid to be delivered and the sick and wounded brought out for treatment.

But on Thursday the Security Council failed to support the proposal.

Washington backed it but Damascus ally Moscow dismissed it as "not realistic.".

The two governments also crossed swords over US-led air strikes that hit forces allied to the Damascus regime in eastern Syria late Wednesday and early Thursday.

US Defence Secretary Jim Mattis said the coalition acted in self-defence after pro-Damascus forces moved on an area under the control of the US-backed Syrian Democratic Forces.

At least 100 pro-regime fighters were killed, a US military official said.

The Syrian foreign ministry condemned the bombardment as a "war crime," an accusation echoed by the Russian ambassador to the UN.

"To confront those who really fight international terrorism on the ground in Syria is criminal," Vassily Nebenzia said.

Wounded fighters were taken to the military hospital in the government-controlled eastern city of Deir Ezzor.

A reporter contributing to AFP saw at least six fighters, lying on hospital beds in sparsely equipped wards.