"Wounded victims are dying only because they can not be treated in time", Gasser said.
The UN chief said he was "deeply saddened by the bad suffering" of civilians in Eastern Ghouta where he said 400,000 people "live in hell on Earth".
"My appeal to all those involved is for an immediate suspension of all war activities in eastern Ghouta allowing for humanitarian aid to reach all those in need", he said.
The United Nations has been pleading with the Syrian government for months to arrange the evacuation of hundreds of seriously ill patients among the population of nearly 400,000. REUTERS/Bassam Khabieh People sit a medical point in the besieged town of Douma, Eastern Ghouta, Damascus, Syria February 20, 2018. "This is the only thing I can say".
However, Assad's veto-wielding ally Russian Federation has called the proposal "not realistic".
The last two days have been the deadliest in years but the Syrian government shows no sign of letting up, vowing to wipe out the rebels in eastern Ghouta, which, like all opposition forces, are dismissed as terrorists by the Assad regime and its allies.
Women and children, trapped for five years of siege, were now in a state of panic, without food, sanitation, or safe havens after air strikes destroyed their homes, Zeid said.
State media reported that two people were wounded on Wednesday as rebels fired mortars on the districts of Damascus near eastern Ghouta.
Winter Weather Advisory: Freezing Rain Possible
Those living outside of the Champlain Valley are under a Winter Weather Advisory through tomorrow morning ( Sunday 7am). But the weekend has the potential to be eventful weather-wise with a combination of freezing rain , sleet and snow .
A spokesman for Russia's Defence Ministry said there had been a "massive bombardment by illegal armed groups from eastern Ghouta" which had targeted residential areas, hotels and Russia's Centre for Syrian Reconciliation. Assad's forces captured Aleppo in December 2016 in his biggest victory since the conflict began in March 2011.
The spark of the recent unprecedented violence was ignited a couple of months ago when al-Qaida-linked groups launched an offensive against a key military base in the city of Harasta in the rebel-held Eastern Ghouta countryside of Damascus, prompting a counter military offensive in that area.
It is also among the last places to resist Assad's determined campaign to take back control of every last rebel-held region.
Eastern Ghouta is one of a group of "de-escalation zones" under a diplomatic ceasefire initiative agreed by Assad's allies Russian Federation and Iran with Turkey, which has backed the rebels. But a former al Qaeda affiliate, which has a small presence there, is not included in the agreement.
Civilians in Ghouta still live in fear after the 2013 chemical attack on the area that shocked the world.
United Nations human rights chief Zeid Ra'ad al-Hussein attacked what he called a "monstrous campaign of annihilation" on Wednesday against the besieged civilian population of eastern Ghouta in Syria.
It was the second straight day that the civilian death toll topped 100, after 127 were killed Monday in Eastern Ghouta's bloodiest day in four years. They were repelled by Turkish shelling.
Meanwhile, President Recep Tayyip Erdogan said Turkey is determined to press ahead with a separate offensive to clear a Syrian enclave of Kurdish fighters despite an attempt by Syrian pro-government forces to enter the region.