Insurgents in southern Syria also asserted themselves on Wednesday, seizing the main border crossing with Jordan, they said; Jordanian officials closed the post. By Wednesday night, Nusra fighters appeared to take credit for the victory, posting photos of themselves at the scene. It was a potential embarrassment for Jordan and the United States, as Syria’s southern front is one of the few places where relatively moderate, nationalist insurgents retain some clout. They receive some Western aid, yet sometimes cooperate with Nusra, listed as a terrorist group.

Under pressure from Jordan, other insurgents reached an agreement with Nusra that it would stay out of the battle, or at least hide its participation, said Abu Mosaab, an antigovernment activist in the area who uses a nickname for safety. (Issam Rayyes, a spokesman for the other groups, gave a different version, saying that they had won the battle, mainly with artillery, only to have Nusra fighters elbow in — and that Nusra would not be allowed a role in running the crossing.)

Several times, residents of the Salamiyeh area say, they have asked for reinforcements, only to have officials remind them that thousands of the area’s young men have failed to report for military duty. Maher, a refugee from the area who gave only a first name for fear of reprisals, said that a delegation including his uncle had even met with President Bashar al-Assad, who told them, “You have 24,000 recruits who avoided the army; let them join.”

Mr. Assad has said in recent interviews that he is fulfilling his responsibilities and protecting the Syrian people.

Islamic State fighters attacked the nearby village of Maboujeh on Tuesday, residents said. A journalist from the area, who asked not to be named out of fear for his safety, said that 48 bodies had been buried on Wednesday, and that residents were angry that the government had not sent “real army, tanks and heavy weapons” to back up lightly armed pro-government militias.