This photo release by the Syrian Red Crescent, shows members of the Syrian Red Crescent distributing medicines for civilians in Douma, eastern Ghouta, a suburb of Damascus, Syria, Monday, March. 5, 2018. Desperate for food and medicine, Syrian civilians in the war-ravaged eastern suburbs of Damascus hoped for relief Monday as a 46-truck aid convoy began entering the rebel stronghold, the first such shipment in months. (Syrian Red Crescent via AP)
This photo release by the Syrian Red Crescent, shows members of the Syrian Red Crescent distributing medicines for civilians in Douma, eastern Ghouta, a suburb of Damascus, Syria, Monday, March. 5, 2018. Desperate for food and medicine, Syrian civilians in the war-ravaged eastern suburbs of Damascus hoped for relief Monday as a 46-truck aid convoy began entering the rebel stronghold, the first such shipment in months. (Syrian Red Crescent via AP)

Beirut — Civilian deaths have surged in a besieged Damascus suburb this week as Syrian government forces press an offensive to recapture the area in defiance of U.N. calls for a cease-fire and increasingly desperate international efforts to halt one of the gravest humanitarian crises of the war.

More than 600 people have died and thousands have been wounded in the rebel-held enclave of Eastern Ghouta since mid-February, according to the United Nations. The shortcomings of a U.N. Security Council resolution urging a cease-fire were clear Monday as the first aid convoy to reach the area this year halted operations amid continued shelling and airstrikes.

At least 100 people are estimated to have been killed in the enclave since Monday, marking one of the deadliest spells since the military operation began.

Eastern Ghouta is the final rebel pocket on the outskirts of Damascus, the capital. As Syrian government and allied Russian forces wage their relentless campaign to reclaim the area, rebel groups have sent volleys of mortar shells into densely populated districts of Damascus.

As many as 400,000 civilians are caught in the middle in Eastern Ghouta, many severely weakened after five years of siege. Shortly before midnight on Monday, local doctors said they had treated at least 12 patients in the Hamouriyeh district with breathing difficulties consistent with exposure to chlorine munitions. Those reports could not immediately be verified, and the Syrian government has also used fertilizers in its bombs, which produces similar symptoms.

Hamouriyeh sits in the center of the enclave and appears to be a key target for forces loyal to President Bashar Assad as they move to split Eastern Ghouta in half. A team of U.N. war-crimes investigators has blamed Syrian forces for earlier chlorine attacks in Eastern Ghouta, which caused patients to faint, vomit and suffer blurred vision.

The Syrian army’s advances continued on Tuesday as troops chipped away at rebel-held areas east and west of Ghouta. The front page of the government-aligned al-Watan newspaper showed pictures of U.N. aid trucks moving through empty streets controlled by Syrian soldiers.