Syrian regime jets pounded Eastern Ghouta on Thursday, sending the death toll for the four-day assault on the rebel enclave on the outskirts of Damascus soaring past 200.
Violence also flared in the east of Syria, where the US-led coalition said it killed at least 100 pro-regime fighters to fend off an attack on its Kurdish allies.
The clash marked a fresh escalation between Washington, which has threatened the regime over its alleged use of chemical weapons, and Damascus, which labelled the latest incident a "war crime".
The United States, for its part, demanded a ceasefire in Eastern Ghouta. "These attacks must end now," State Department spokeswoman Heather Nauert said.
In Eastern Ghouta, which lies east of the capital and has been besieged since 2013, residents had no time to mourn their dead or treat their wounded from the previous day of bombardment.
"These are the worst four days that Eastern Ghouta has ever gone through," said Hamza, an overwhelmed doctor at the local Erbin clinic who was treating wounded patients.
"From 2011 until now, there has never been the level of bombardment we've seen in the last 96 hours."
According to the Syrian Observatory for Human Rights, at least 58 people were killed on Thursday in strikes on several locations in Eastern Ghouta.
- 53 children -
That brought to 211 the number of civilians killed since the regime launched a campaign Monday of heavy air raids on the area, which has an estimated 400,000 residents.
The Observatory said 53 of the victims were children.
With few bulldozers and precious little fuel to operate them, rescue workers have struggled to reach trapped civilians and are having to work under fire.
At the hospital in Erbin, one of the places in Ghouta which was repeatedly hit this week, an AFP reporter saw the bodies of adults and children lined up on the floor.
Nurses at the medical facility cried over the body of one man who had been working as a guard at the hospital.
At least two civilians were killed in retaliatory rebel mortar fire on government-controlled areas of Damascus, according to state news agency SANA.
Eastern Ghouta was one of several so-called de-escalation zones agreed last year by three of the main outside players in the conflict -- Turkey, Iran and Russia.
Ankara announced Thursday it would host a new three-way summit to revive efforts to end the war, which has killed at least 340,000 people and displaced millions since 2011.
Recent attempts to bring the seven-year conflict's protagonists and brokers to the table have floundered and the demise of the Islamic State group's "caliphate" late last year has seen growing tensions between the various forces that once all fought against the jihadists.
A US military official said the US-led coalition that still assists Kurdish-led forces in the hunt for surviving IS members in eastern Syria killed at least 100 pro-regime fighters overnight.
- Oil and gas -
The US Central Command said the coalition acted in self-defence after pro-government forces moved on an area under the control of the Kurdish-led Syrian Democratic Forces.
The SDF and the coalition targeted the attacking forces with air strikes and shelling after "20 to 30 artillery and tank rounds landed within 500 metres (yards) of the SDF headquarters location", a US official said.
Syrian state media confirmed that dozens were killed in the clash but appeared to deny the forces were army soldiers, describing the victims as "popular forces".
The head of the Britain-based monitoring group, Rami Abdel Rahman, said the aim of the attack appeared to be the capture of a key oil field and a major gas plant in an SDF-held area.
The Omar oil field, one of the biggest in Syria, had a pre-war output of 30,000 barrels per day, while the Conoco gas field had a pre-war capacity of 13 million cubic metres a day.
According to the Observatory, the forces that launched the attack on SDF positions were local tribal fighters loyal to Assad and Afghan Shiite militia fighting alongside the regime.
In a letter addressed to the UN secretary general, the Syrian foreign ministry said the attack "represents a war crime and a crime against humanity".
It accused the United States of trying to set up permanent bases in Syria under the cover of fighting terrorism.
"The Syrian government renews its call to dissolve this illegitimate coalition," it said.