Fifteen children between the ages of six months to eighteen months have died in northern Syria after they were accidentally given an anesthetic instead of a measles vaccine, according to media reports published Wednesday. The immunization program has been suspended while authorities conduct an inquiry into the deaths.

The incident occurred in the city of Idlib in northwestern Syria, where medics from international aid agencies were vaccinating children as part of a measles immunization campaign, according to a joint statement released by the World Health Organization, or WHO, and UNICEF.

A preliminary investigation conducted shortly after the incident, by the health care directorate of the Syrian opposition's interim government in Idlib, found that a drug named Atracurium, which is used as a muscle relaxant during surgeries, was given to nearly 75 infants. The drug is usually fatal when given to infants but older children can survive low dosages due to a higher body weight. The error is believed to have occurred due to similar packaging being used for the anesthetic and the measles vaccine.

“The investigation is continuing to find out who is responsible,” Mohammed Saad, a health care directorate official, told Reuters, adding that the investigation was now focusing on clinical negligence.

WHO, in its statement, said that it had “assigned a team of experts to provide assistance in investigating this event” and that it was also providing “advice and protocols” for the investigation.

“For as long as the facts remain unclear, the suspension of the immunization campaign in both Idlib and Deir Ezzour provinces is a wise step,” the statement said. “However, it is vital that immunization efforts against measles -- a disease which is a leading killer of children worldwide -- resume in Syria as soon as possible.”

The conflict between rebels and Bashar Assad’s forces, which is now in its fourth year, has led to the collapse of the health care system in large parts of Syria. A number of children have been affected by diseases like polio and measles as vaccination efforts, led by aid agencies, have been thrown into disarray.