Doctors in Syria's rebel-controlled suburbs of Damascus have said they cannot keep up with the number of casualties, amid shelling by government forces.
The bombing campaign has targeted hospitals, apartment blocks and other civilian sites, killing and wounding hundreds of people in recent days.
The bombardment has forced many among the nearly 400,000 residents to sleep in basements and makeshift shelters, and has overwhelmed rescue workers who have spent days digging out survivors from the wreckage of bombed buildings.
UN Secretary-General Antonio Guterres called for an immediate suspension of "all war activities" in the rebel-held Damascus suburbs known as eastern Ghouta where he said 400,000 people are living "in hell on earth".
The UN chief said a suspension of fighting must allow for humanitarian aid to reach all in need and the evacuation of some 700 people needing urgent medical treatment.
Bishop Declan Lang, Chair of the Bishops' Conference Department for International Affairs, has called the killing of civilians in Eastern Ghouta a "grievous offence to our common humanity".
He said: "Their suffering underscores how over seven years of conflict in Syria, the international community has failed to protect the innocent.
"In this dark moment I urge Catholics in England and Wales to pray for the people of Eastern Ghouta and Syria; to encourage our own government to help end the violence; and to support generously those humanitarian agencies that are bravely working to save lives."
The UN human rights office said in a statement on Wednesday that at least 346 people had been killed in eastern Ghouta since the Syrian government and its allies escalated their offensive on the region on 4th February.
At least 92 of those deaths occurred in just one 13-hour period on Monday, it said, adding that the toll was far from comprehensive, documented in the midst of chaos and destruction. Another 878 people have been wounded, mostly in air strikes hitting residential areas.
Sweden and Kuwait have called for a vote on a UN resolution ordering a 30-day cease-fire throughout Syria to enable delivery of humanitarian aid to millions of people in acute need, and the evacuation of the critically sick and wounded.
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