Pro-government forces pounded the rebel-held district of eastern Ghouta district on Tuesday, in a surge of violence which a war monitor said has killed at least 250 people since Sunday night.
The Britain-based Syrian Observatory for Human Rights said it was the highest 48-hour death toll in the Syria conflict since a 2013 chemical attack on eastern Ghouta, the last major rebel bastion located near the capital Damascus.
The wave of air strikes, rocket fire and shelling has sparked international condemnation. France, a permanent member of the UN Security Council, described the bombardment as a serious violation of international humanitarian law.
There was no immediate comment from the Syrian military. Damascus says it only targets militants.
Recent violence in the besieged suburb is part of a wider surge in fighting on several fronts as President Bashar al-Assad's military pushes to end the seven-year rebellion against him.
The Observatory said the bombardment had killed more than 106 adults and children in eastern Ghouta on Tuesday alone.
Panos Moumtzis, the United Nations' humanitarian coordinator for Syria, on Tuesday condemned the bombing of five hospitals in eastern Ghouta and said intentional attacks on medical facilities "may amount to war crimes".
In Brussels, Syrian opposition leader Nasr al-Hariri - a delegation head at stalled UN peace talks - pleaded for more international pressure on Assad to stop.
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In Geneva, the UN children's agency expressed outrage at the casualties among the enclave's children, saying it had run out of words.
Those killed since the escalation began on Sunday include 58 children. Another 1,200 people have been injured in the bombardment, the Observatory said.
Rescuers said the air raids created "a state of terror" among residents in eastern Ghouta, where the UN says nearly 400,000 people live. The pocket of satellite towns and farms has been under government siege since 2013.
Factions in Ghouta fired mortars at Damascus on Tuesday, killing six people and injuring 28, Syrian state TV said. The army retaliated and pounded militant targets, according to the state news agency SANA.
The Syrian foreign ministry accused militants in Ghouta of targeting Damascus and using people as "human shields". It said in a letter of complaint to the United Nations that some Western officials were denying the government's right to defend itself.
The Civil Defence in eastern Ghouta, a rescue service that operates in rebel territory, said jets battered Kafr Batna, Saqba, Hammouriyeh and several other towns on Tuesday.
"The warplanes are not leaving the sky at all," said Siraj Mahmoud, a civil defence spokesman in Ghouta, as the sound of explosions rang out in the background.
Mahmoud said that government forces bombed houses, schools and medical facilities, and that rescuers had found more than 100 people dead "in one day alone" on Monday.
Reuters photos showed bandaged people waiting at a medical point in the town of Douma, some of them with blood streaming down their faces and their skin caked in dust.
Bombs struck five hospitals in the enclave on Monday, said the UOSSM group of aid agencies that funds medical facilities in opposition parts of Syria.
Assad's most powerful backer, Russia, has been pushing its own diplomatic track which resulted in establishing several "de-escalation zones" in rebel territory last year.
Fighting has raged on in eastern Ghouta even though it falls under the ceasefire plans that Moscow brokered with the help of Turkey and Iran. The truces do not cover a former Al Qaida affiliate, which has a small presence in the besieged enclave.
Residents and aid workers say the "de-escalation" deals have brought no relief. Food, fuel and medicine have dwindled.
The two main rebel factions in eastern Ghouta, which signed the deals with Russia last summer, accuse Damascus and Moscow of using the jihadist presence as a pretext for attacks.
Moscow did not comment on the renewed bombing in eastern Ghouta on Tuesday.
On Monday Russian Foreign Minister Sergey Lavrov blamed "armed provocations" by Nusra militants, formerly linked to Al Qaida, for conditions in Ghouta. He said Moscow and its allies could "deploy our experience of freeing Aleppo ... in the eastern Ghouta situation".
UN Syria envoy Staffan de Mistura warned on Tuesday that the escalating battle in Ghouta could turn into a repeat of the bloody fight for Aleppo, over which Damascus regained full control in late 2016 after years of fighting.