DAMASCUS, Syria—Government forces are tightening the noose around one of the suburbs gassed by chemical weapons in August, raising concerns of a fresh humanitarian crisis as residents forage for olives, grapevine leaves and other basic foods.

Pro-regime fighters have encircled about 12,000 people, mostly civilians but also including some rebel fighters, in the town of Moadhamiya, according to local and international aid workers, opposition activists and people interviewed on Monday in a government-controlled section of the town.

Moadhamiya, located about 7 miles from central Damascus and visited by a Wall Street Journal reporter, has been inaccessible to Western media since the start of the year.

"We won't allow them to be nourished in order to kill us," said a 24-year-old pro-regime paramilitary in the government-controlled section of Moadhamiya, referring to rebels and their supporters just a few hundred yards away on the other side of town. "Let them starve for a bit, surrender and then be put on trial."

The town has been under siege by government forces since April. But pressure has intensified since chemical weapons bearing the nerve agent sarin hit the area Aug. 21, residents say. The chemical attack, which was confirmed in a report issued last month by a United Nations team of inspectors who visited the town on Aug. 26, killed about 80 people in the town's rebel-held section, according to activists. More than 1,300 were killed in the eastern suburbs of Damascus as part of the same attack.