A convoy of Syrian Red Crescent trucks arrives in Douma in the rebel enclave of Eastern Ghouta on March 5, 2018, on the outskirts of Damascus. (Amer Almohibany/AFP/Getty Images)

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 periods 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 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 be immediately verified; the Syrian government also has used fertilizer 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 al-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 people to faint, vomit and suffer blurred vision

The Syrian army’s advances continued 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. “Army controls Mohamedeya, nears Douma and besieges Harasta completely,” read one headline, reeling off a list of districts in the area.

The Trump administration has considered new military action against the Syrian government in response to allegations of ongoing chemical weapons use, The Washington Post reported Monday, raising the prospect of a second U.S. strike on Assad in less than a year.

But there appears to be little appetite for deeper intervention, as the United States wrestles with the political contradictions of its military deployments in Kurdish-controlled areas across Syria’s north and east. On Tuesday, a U.S.-backed alliance known as the Syrian Democratic Forces, which was formed to fight the Islamic State militant group, said it was diverting 1,700 troops to a separate front in the north where Kurdish militants are clashing with Turkey, one of Washington’s NATO allies.

More broadly, the international community has failed to translate fierce rhetoric at the Security Council into results after the Feb. 24 cease-fire resolution aimed at halting the fighting in Eastern Ghouta and ensuring safe passage for aid convoys there.

Food parcels, nutritional supplements and some medical supplies arrived aboard a 46-truck U.N. convoy that reached Eastern Ghouta on Monday amid fraught international negotiations and increasingly desperate pleas from trapped civilians.

But by nightfall, dozens of civilians were dead and the aid mission had been forced to pull out. It was unclear when or whether the U.N.-led aid convoys could resume.

“We kept going until the situation had escalated so much that we could not guarantee that our teams would be safe,” said Pawel Krzysiek, a spokesman for the International Committee of the Red Cross in Syria. The United Nations said that more than a quarter of the trucks had yet to be fully emptied. Some had not been opened. 

In Eastern Ghouta’s hospitals, supplies have been exhausted and wounds are bandaged with rags. On the busiest days, doctors have been able to save only those patients with the greatest chance of pulling through. The most grievously wounded have died without painkillers. 

The government had stripped Monday’s convoy of much of its medical supplies, including surgical kits, insulin and dialysis equipment, U.N. officials said.

“We delivered as much as we could amid shelling. Civilians are caught in a tragic situation,” Sajjad Malik, a representative for the U.N. displacement agency in Syria, wrote on Twitter late Monday.

Erin Cunningham and Zakaria Zakaria in Istanbul contributed to this report.