DEIR SONBUL, Syria — The government of Syria, trying to contain a rapidly expanding insurgency, has resorted to one of the dirty tricks of the modern battlefield: salting ammunition supplies of antigovernment fighters with ordnance that explodes inside rebels’ weapons, often wounding and sometimes killing the fighters while destroying many of their hard-found arms.

The practice, which rebels said started in Syria early this year, is another element of the government’s struggle to combat the opposition as Syria’s military finds itself challenged across a country where it was not long ago an uncontested force. The government controls the skies, and with aircraft and artillery batteries it has pounded many rebel strongholds throughout this year. But the rebels continue to resist, mostly with small arms.

Doctored ammunition offers an insidious way to undermine the rebels’ confidence in their ammunition supply while simultaneously thinning their ranks.

“When they do this, you will lose both the man and the rifle,” said Ghadir Hammoush, the commander of a fighting group in Idlib Province who said he knew of five instances in which rifles had exploded from booby-trapped ammunition.