"It's imperative to end this senseless human suffering now. Such targeting of innocent civilians and infrastructure must stop now," Panos Moumtzis, the UN's Regional Humanitarian Coordinator for the Syria Crisis said in a statement.
Several days of bombardment on Eastern Ghouta, near Damascus, has killed hundreds of people, with one monitor of Syria's seven-year conflict saying that at least 77 people had died there yesterday alone.
Held by rebels since 2012, Eastern Ghouta is the last opposition pocket around Damascus and President Bashar al-Assad has dispatched reinforcements there in an apparent concerted effort to retake it.
"The recent escalation of violence compounds an already precarious humanitarian situation for the 393,000 residents of East Ghouta, many of them internally displaced," he said.