BEIRUT, Lebanon — Eyes watering, struggling to breathe, Abd al-Mouin, 22, dragged his nephews from a house reeking of noxious fumes, then briefly blacked out. Even fresh air, he recalled, was “burning my lungs.”

The chaos unfolded in the Syrian town of Sarmeen one night this spring as walkie-talkies warned of helicopters flying from a nearby army base, a signal for residents to take cover. Soon, residents said, there were sounds of aircraft, a smell of bleach and gasping victims streaming to a clinic.

Two years after President Bashar al-Assad agreed to dismantle Syria’s chemical weapons stockpile, there is mounting evidence that his government is flouting international law to drop jerry-built chlorine bombs on insurgent-held areas. Lately, the pace of the bombardments in contested areas like Idlib Province has picked up, rescue workers say, as government forces have faced new threats from insurgents. The Assad government has so far evaded more formal scrutiny because of political, legal and technical obstacles to assigning blame for the attacks — a situation that feels surreal to many Syrians under the bombs, who say it is patently clear the government drops them.

“People are so used to it, they know from the sound,” said Hatem Abu Marwan, 29, a rescue worker with the White Helmets civil defense organization, a note of exasperation creeping into his voice when asked to explain. “We know the sound of a helicopter that goes to a low height and drops a barrel. Nobody has aircraft except the regime.”