An improperly mixed or possibly sabotaged measles vaccine has killed as many as 50 children in insurgent-held areas of northwestern Syria, volunteer medical organizations and physicians reported Wednesday, forcing the suspension of a large-scale United Nations vaccination campaign intended to stop the spread of measles, rubella and polio.

The victims, some of them infants, apparently all died Tuesday, mostly concentrated in the cities of Jarjanaz and Sinjar in Idlib Province, an area controlled by forces opposed to President Bashar al-Assad. These opposition forces have sought to function as an interim government and provide basic health care, including inoculations and vaccinations to children, given the collapse of Syria’s public health care system since the civil war began more than three years ago.

There were conflicting accounts on the precise number of deaths, with some doctors reporting at least 15, and others saying 35 or perhaps 50. Dozens of other children in Idlib were reported to have been sickened by the suspect vaccine.

Dr. Abdulla Ajaj, a physician who helped administer the vaccine, said the suspect batch of doses had been received three days before they were used. “This is the first time we have had such a problem,” he said in a Skype interview.