GENEVA — 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 on 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 al-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.