Fresh bombardment on Eastern Ghouta killed dozens Thursday, bringing the number of dead civilians in a five-day assault by the Syrian government to more than 400.

Mounting calls for a humanitarian truce in one of the bloodiest episodes of Syria's seven-year conflict failed to stop 46 more people being killed by air strikes and rocket fire.

Regime backer Russia said there was "no agreement" at the UN Security Council on a 30-day ceasefire for Syria and presented amendments to a draft resolution that would allow aid deliveries and the evacuation of civilians from besieged Eastern Ghouta.

As diplomats wrangled over a UN vote, which could take place on Friday, people huddled in basements while government forces pounded the enclave with rockets and bombs, turning towns into fields of ruins and even hitting hospitals.

Aid group Doctors Without Borders said 13 of the facilities it supports in Eastern Ghouta were damaged or destroyed in three days, leaving remaining staff with very little to save the hundreds of wounded brought to them every day.

In the hospital mortuary in Douma, the main town in the enclave just east of Damascus, bodies wrapped in white shrouds were already lined up on the floor, two of them children.

Nowhere safe

"Five days of air strikes and intense artillery fire by the regime and its Russian ally have killed 403 civilians, including 95 children," said the Syrian Observatory for Human Rights.

Morning rain appeared to initially keep warplanes away on Thursday but the sky cleared by midday and jets, some of them Russian according to the Observatory, soon returned.

Russia has so far denied direct involvement in the assault on Ghouta but the pro-government Syrian newspaper Al-Watan reported on Thursday that Russian warplanes and advisers had joined the battle.

The US said on Thursday Russia had "unique responsibility" for the deaths.

"Without Russia backing Syria, the devastation and the deaths would certainly not be occurring," State Department Spokeswoman Heather Nauert told journalists.

Regime and allied forces have been massing around the enclave, in which an estimated 400,000 people live, ahead of a likely ground offensive to flush out holdout Islamist and jihadist groups.

"We are 14 women and children living in a room that is 10 feet wide, with no toilet and nowhere to wash," said 53-year-old Umm Abdo, who joined a large group in the basement of a school in Arbin.

The brief respite provided by the rain on Thursday encouraged some residents to venture out of their basements and shelters, to buy food, check on their property or enquire about their relatives and neighbours.

In the town of Hammuriyeh, a queue had formed outside a shop as starving residents tried to stock up but another rocket sowed panic and sent everybody back to their shelters.

In Douma, a young boy tried to peddle lighters on the street but rocket fire quickly forced him to scamper back to cover.

• Powerless -

An AFP correspondent saw rescuers known as the "White Helmets" forced to abandon efforts to retrieve a wounded woman from the rubble of a collapsed home when air strikes resumed.

When they ventured back to the site, the woman was dead.

"Five days of air strikes and intense artillery fire by the regime and its Russian ally have killed 403 civilians, including 95 children," said the Syrian Observatory for Human Rights.

Morning rain appeared to initially keep warplanes away on Thursday but the sky cleared by midday and jets, some of them Russian according to the Observatory, soon returned.

Russia has so far denied direct involvement in the assault on Ghouta but the pro-government Syrian newspaper Al-Watan reported on Thursday that Russian warplanes and advisers had joined the battle.

The US said on Thursday Russia had "unique responsibility" for the deaths.

"Without Russia backing Syria, the devastation and the deaths would certainly not be occurring," State Department Spokeswoman Heather Nauert told journalists.

Regime and allied forces have been massing around the enclave, in which an estimated 400,000 people live, ahead of a likely ground offensive to flush out holdout Islamist and jihadist groups.

"We are 14 women and children living in a room that is 10 feet wide, with no toilet and nowhere to wash," said 53-year-old Umm Abdo, who joined a large group in the basement of a school in Arbin.

The brief respite provided by the rain on Thursday encouraged some residents to venture out of their basements and shelters, to buy food, check on their property or enquire about their relatives and neighbours.

In the town of Hammuriyeh, a queue had formed outside a shop as starving residents tried to stock up but another rocket sowed panic and sent everybody back to their shelters.

In Douma, a young boy tried to peddle lighters on the street but rocket fire quickly forced him to scamper back to cover.

• Powerless -

An AFP correspondent saw rescuers known as the "White Helmets" forced to abandon efforts to retrieve a wounded woman from the rubble of a collapsed home when air strikes resumed.

When they ventured back to the site, the woman was dead.

Talks for a deal between the regime and the armed groups controlling Ghouta appear to have stalled.

Russian Foreign Minister Sergei Lavrov said jihadist fighters in Ghouta had rejected an evacuation deal.

"A few days ago, our military in Syria suggested to the fighters that they withdraw peacefully from Eastern Ghouta, like the evacuation of fighters and their families that was organised in East Aleppo," he said.

The head of the defence committee in Russia's lower house of parliament said Thursday that more than 200 new types of weapons were tested as part of his country's military support to the regime of President Bashar al-Assad.

"It's not an accident that today they are coming to us from many directions to purchase our weapons, including countries that are not our allies," he said.