Concerns are growing that Syria faces the worst humanitarian catastrophe of its seven-year civil war after more than 250 civilians were reportedly killed in airstrikes by forces loyal to Bashar al-Assad within two days.
Amnesty International said there were "flagrant war crimes" going on in eastern Ghouta, near the capital Damascus.
Seven hospitals were bombed by the Russian-backed forces since Monday morning (19 February) with two hospitals suspending operations and one being put out of service.
At least 50 children are believed to have died with 1,000 maimed and wounded.
"We are standing before the massacre of the 21st century," said Diana Semaan, from Amnesty International.
"If the massacre of the 1990s was Srebrenica, and the massacres of the 1980s were Halabja and Sabra and Shatila, then eastern Ghouta is the massacre of this century right now," Semann told the Guardian.
Opposition activists said government forces have brought in more reinforcements, signalling major assault to recapture the last main rebel stronghold in the city which is home to 400,000 people.
It also hosts thousands of insurgents belonging to different factions such as the Army of Islam and Failaq al-Rahman, with a small presence of al-Qaida-linked fighters.
The Syrian Observatory for Human Rights said this was the deadliest period in eastern Ghouta since 2015 and that 194 people had died in the last 48 hours.
Ahmad al-Dbis, from the Union of Medical Care and Relief Organisations said ten hospitals have been damaged by airstrikes or shelling in the last 48 hours.
"From this morning, this early morning, continuous shelling, airstrikes is going on. So civilians in eastern Ghouta are frightened.
"It's very hard life here, it's a miserable life. We want the world to demonstrate credibility about human rights. We want the the world to stop this crime against civilians in eastern Ghouta," a doctor at one of the hospitals told ITV News.
The International Committee of the Red Cross issued a statement that "this cannot go on", adding "Dozens of mortars in Damascus cause civilian casualties and spread fear. We cannot let history repeat itself".