BEIRUT, Lebanon — For two days and a night, the computer science student had been huddling with his family in the basement of their apartment building as pro-government forces rained bombs down on their rebel-held Syrian town.

After night fell, they heard the whirring of helicopter blades followed by the whistling sounds of objects falling from the sky. Soon, a strange smell wafted down the stairs.

“People started shouting in the streets, ‘Chemicals! Chemicals!’” the student, Mohammed al-Hanash, 25, said by phone from Syria.

The attack in the Syrian town of Douma on Saturday, which witnesses and medical workers said used chemical weapons, has resonated far beyond the war-scarred community’s destroyed buildings, ratcheting up tensions among world powers and threatening to escalate Syria’s multi-sided civil war.