The Red Cross demanded access Wednesday to an opposition-held enclave near Damascus where Assad regime jets have been raining bombs in a four-day blitz that has killed more than 300 civilians.
The United Nations chief Antonio Guterres described the death and devastation that has engulfed Eastern Ghouta since Sunday as "hell on earth" while France called for an immediate truce.
Fresh air raids on several parts of the enclave killed at least 40 civilians and wounded 350.
Many of the region's hospitals have also been targeted, leaving many out of service.
The International Committee of the Red Cross said the situation was so dire that its teams should be allowed access to eastern Ghouta to help overwhelmed doctors and nurses.
"The fighting appears likely to cause much more suffering in the days and weeks ahead, and our teams need to be allowed to enter eastern Ghouta to aid the wounded," said Marianne Gasser, ICRC's head of delegation in Syria.
Regime warplanes have this month intensified their strikes on the area, which lies just east of Damascus and is home to around 400,000 civilians.
"Wounded victims are dying only because they cannot be treated in time," Gasser said.
According to the Syrian Observatory for Human Rights, around 1,500 people have been wounded since Sunday in the one of the bloodiest episodes of Syria's seven-year-old war.
Many of Wednesday's victims were killed when barrel bombs -- crude, improvised munitions that usually cause indiscriminate damage -- were dropped on the town of Kfar Batna.
The hospital in Douma, the largest town in eastern Ghouta, is still functioning but the influx of wounded is such that doctors and nurses cannot save everyone.
The U.N. and an NGO said at least seven hospitals were bombed on Monday and Tuesday.
"The regime claims that it's targeting armed groups and terrorists, but the truth is that it only targets civilians," said Doctor Ahmad Abdulghani, at he Dar al-Shifa hospital in Hammuriyeh, heavily damaged by a strike.
"This is a hospital that treats the sick and civilians, why target it?"
Observatory head Rami Abdel Rahman said at least 72 children were among the 312 people killed in strikes since Sunday.
Assad regime news agency SANA said 18 people were wounded in Damascus by rebel shelling.
Guterres told the U.N. Security Council he was "deeply saddened by the terrible suffering" of civilians in eastern Ghouta where he said people "live in hell on Earth."
He called for an immediate humanitarian truce, as did French President Emmanuel Macron who accused Bashar al-Assad's regime of using terrorism as a "pretext" to attack civilians.
Russia, which is Assad's main backer but denies direct involvement in the Ghouta strikes, called for a Security Council meeting on the issue.
Anti-regime groups have controlled the area since 2012.
With the Daesh terror group's once sprawling "caliphate" now wiped of the map, the regime looks bent on completing its reconquest.
Ghouta, on the outskirts of the capital, is a key target.
In recent days, government forces have been massing around eastern Ghouta, apparently preparing for a ground offensive.
"We have long feared eastern Ghouta will see a repeat of the terrible scenes observed by the world during the fall of east Aleppo and these fears seem to be well founded," said Mark Schnellbaecher, the regional head of the International Rescue Committee.
The battle that saw government forces wrest back the country's second city from opposition forces in 2016 caused extensive destruction and suffering that drew comparisons with the World War II era devastation in Stalingrad and the Warsaw ghetto.
"We don't dare go outside," said Khadija, 53, hiding in a basement in the town of Douma.
Shivering as she gathered several children around her in the darkened room, she said: "There are planes above us and missiles landing around us: where should we go with our children?"