– More than 133,000 people have fled a suburb of the Syrian capital, Damascus, in the face of the government’s military onslaught, the United Nations refugee agency said Tuesday, as the United States and its allies weighed how to respond to a suspected chemical attack there.

The exodus from the suburb, eastern Ghouta, the last major rebel-held pocket near the capital city, has come after two months of ferocious bombardment by the Syrian military and by its Iranian and Russian allies, as the government of President Bashar Assad has retaken control of most of the region. It is estimated that more than 1,600 people have been killed in the campaign.

On Sunday, groups in Douma, a town in eastern Ghouta, reported what appeared to be a chemical weapons attack by the government, though Russian and Syrian officials have denied that chemical agents were used. Experts and Western officials say that multiple times in its seven-year civil war, Syria has used chemical weapons against its own people, including a sarin gas attack in Ghouta in 2013 that by some estimates killed more than 1,000 people.

President Donald Trump has promised a “forceful” response to the latest episode. At the U.N. Security Council on Tuesday, Russia vetoed a U.S.-backed proposal to create a new body to investigate chemical weapons attacks and determine blame for them.

The wave of refugees from eastern Ghouta, an area with hundreds of thousands of civilians, is the latest in a war that has displaced millions of people, and the office of the U.N. High Commissioner for Refugees voiced alarm at the surging numbers of casualties and people driven from their homes. Tens of thousands of civilians remain trapped, and relief agencies are just beginning to grasp the full extent of the humanitarian needs.

“Only a monster targets civilians and then ensures that there are no ambulances to transfer the wounded, no hospitals to save their lives, no doctors or medicine to ease their pain,” Nikki Haley, the U.S. ambassador to the United Nations, said on Monday.

Mohammad Javad Zarif, Iran’s foreign minister, called the U.S. position hypocritical, noting that the United States supported Iraq in the 1980s when it used chemical weapons against its own population and against Iran.

Relief agencies are struggling to cope with the needs of those fleeing eastern Ghouta, including more than 45,000 now packed into eight shelters around the outskirts of Damascus, where the crowded conditions and lack of sanitation pose a threat to health.

Andrej Mahecic, a senior spokesman for the U.N. refugee agency, said it was shipping in tents and supplying a range of other basic supplies, but that many people had fled eastern Ghouta with nothing but the clothes they were wearing. He said the agency was buying garments from local suppliers.

Relief agencies are grappling with similar pressures in the north of Syria, assisting more than 137,000 people who fled Afrin in recent weeks as Turkish troops mounted an offensive to drive out the Kurdish forces controlling it.