In a surge of attacks launched by the Syrian government and its allies, over 100 people were killed in the rebel-held pocket of eastern Ghouta, near the capital Damascus, in the space of 24 hours, a group monitoring the civil war said on Monday.
Another 325 people were wounded in airstrikes, rocket fire and shellings in the besieged suburbs of Damascus, the Syrian Observatory for Human Rights said.
There was no comment from the Syrian military. The Damascus regime has insisted it only targets militants.
Rebel factions in Ghouta fired mortars at districts of Damascus, killing a child and wounding eight others, Syrian state media said. Troops and allied forces struck militant targets there in response, the state news agency SANA said.
The United Nations says nearly 400,000 people live in eastern Ghouta, a pocket of satellite towns and farms under government siege since 2013.
Panos Moumtzis, U.N. regional coordinator for the Syria crisis, said an "extreme escalation in hostilities" killed at least 40 civilians and wounded more than 150 others on Monday.
"The humanitarian situation of civilians in eastern Ghouta is spiraling out of control," he said in a statement. "Many residents have little choice but to take shelter in basements and underground bunkers with their children."
The British-based Observatory said the latest escalation started on Sunday, adding that 18 children were among the dead.
The local civil defense group said warplanes and artillery had pounded Saqba, Jisreen and other towns. The rescue service, which operates in rebel territory, said strikes killed 20 people and wounded dozens in the town of Hammouriyeh alone on Monday.
Syrian President Bashar Assad's military gained momentum in the war after Russian intervention in 2015 tipped the scales in his favor. This turning point in the now nearly seven-year-long war pushed rebels out of major cities, and allowed Assad's forces to retake much of central and eastern Syria from the Islamic State group.
Wael Olwan, spokesman for the Failaq al-Rahman rebel group in eastern Ghouta, said there had been heavy bombing throughout the day.
"There are no ground invasions or clashes, but there is very big shelling and preparatory fire," he said.
Last week, the United Nations said Syria was seeing some of the worst fighting of the war. The multi-sided conflict has killed hundreds of thousands of people and driven millions from their homes.
Malnutrition has increased sharply in eastern Ghouta with hardly any food aid reaching the residents, the U.N. Office for Humanitarian Affairs said.
Rising violence reportedly prompted some 15,000 civilians to flee their homes last month, taking refuge in makeshift shelters or basements in the enclave, OCHA said.
Eastern Ghouta falls under ceasefire plans for rebel territory that Russia has already brokered with the help of Turkey and Iran.
Residents and aid workers, however, say the "de-escalation" deals have brought no relief. Food, fuel, and medicine have dwindled.
Russian Foreign Minister Sergey Lavrov said on Monday that Moscow and its allies could draw on "our experience of freeing Aleppo ... in the eastern Ghouta situation."
With support from Russia and Iran, Damascus regained full control of the city of Aleppo in late 2016, after years of fighting and months of siege ended with an insurgent retreat.
Lavrov blamed "armed provocations" by Nusra militants, formerly linked to al-Qaida, for current conditions in eastern Ghouta.
The two main factions in Ghouta, which include Failaq al-Rahman, accuse Moscow and the Syrian army of breaching the de-escalation agreements. They say they have used the presence of a few hundred fighters from al-Qaida's former Syria branch as a pretext to attack the enclave.
