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.
Syrian forces stepped up an offensive to retake the area earlier this month.
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.
Worst bombing in years
Analysis by Lina Sinjab, BBC Middle East correspondent
The attacks on the Eastern Ghouta area since Sunday have hit not only civilians but also their means of survival, targeting bakeries, warehouses and anything else that may hold food supplies.
It is the worst single day of bombing that people there have seen in years. People are fearful of it becoming another Aleppo scenario.
Aid workers say the attacks targeted major roads in the area, which will block any aid or rescue operation and hinder the movement of ambulances.
The death toll is rising because medical facilities were also hit. Four makeshift hospitals, including a maternity facility, were struck on Monday. The rebels have been responding with mortar attacks on Damascus but the government’s military might is far stronger.
How bad is the situation in the Eastern Ghouta?
Monday’s reported death toll has not been independently verified.
Videos from Hamouria, a town in the enclave where at least 20 people were reportedly killed in air strikes on Monday, showed people fleeing heavily damaged buildings covered in dust and debris.
In December international aid organisations warned conditions in the rebel-held area had reached a “critical point” for civilians because of shortages of food, fuel and medicines.
UN regional humanitarian co-ordinator Panos Moumtzis said it was “imperative” to end the “senseless human suffering” in the Eastern Ghouta.
“Many residents have little choice but to take shelter in basements and underground bunkers with their children,” he noted.
However, Russian Foreign Minister Sergei Lavrov said conditions in the enclave were being exaggerated by international actors.
“In the UN, the topic of humanitarian problems in the Eastern Ghouta and Idlib is being actively hyped up,” he said, according to Russian media reports.
Next month marks seven years of civil conflict in Syria.
Hundreds of thousands of people have been killed and an estimated five million have fled the country.
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.