Bombardments by Syrian government forces have killed scores of civilians in the rebel-held Eastern Ghouta area outside Damascus, monitors say.
If confirmed, it would make Monday one of the deadliest days for the district since it came under siege in 2013.
The UK-based Syrian Observatory for Human Rights (SOHR) said at least 100 civilians, including 20 children, were killed by rockets and air strikes.
The SOHR said 470 people had been injured, some critically.
Urging a halt to the bombardment, a UN official said the situation was spiralling out of control.
The Eastern Ghouta is the last remaining opposition-held enclave near the capital and is completely surrounded by areas under government control.
Last week the Eastern Ghouta, home to almost 400,000 people, received its first aid delivery in almost three months.
What is the Eastern Ghouta?
It is an agricultural belt, about 15km (nine miles) east from the city centre.
The area is made up of 22 communities.
It has been designated a "de-escalation zone" by Russia and Iran, the Syrian government's main allies, along with Turkey, which backs the opposition.
Who controls the enclave?
The main rebel groups are the Salafist Jaish al-Islam (formerly known as Liwa al-Islam), al-Rahman Corps, an affiliate of the Free Syrian Army, and also Hayat Tahrir al-Sham, which is made of several smaller groups including former members of Jabhat al-Nusra, which has its roots in al-Qaeda.