Medics carrying out a vaccination programme in rebel-held northern Syria accidentally administered a muscle relaxant to up to 75 children, killing 15 of them, the opposition says.
A preliminary investigation by the National Coalition found atracurium was given to the children instead of a solution used to mix measles vaccines.
The packaging for the drug is said to be similar to that of the solution. All of the children who died on Tuesday were between six and 18 months old. Their parents initially accused the opposition health authorities of not storing the vaccines properly or using out-of-date medication
The National Coalition meanwhile said it suspected a "security breach by vandals" linked to the government of President Bashar al-Assad.
The three-year-old conflict in Syria and the resultant breakdown in state institutions have meant that diseases such as measles and polio have been spreading.
In response, UN agencies and NGOs have launched immunisation campaigns in rebel-held parts of the country's north and east.
The children who died were among tens of thousands living in Idlib and Deir al-Zour provinces who had been given a second round of measles vaccines since Monday.
A doctor at a clinic in the town of Jarjanaz said the infants had exhibited signs of severe shock about an hour after they had been given the injections, with many suffocating to death as their bodies swelled.
On Wednesday evening, the National Coalition said a preliminary investigation had found atracurium, a muscle relaxant used in surgery, was administered.
The error was believed to have occurred because the packaging of the atracurium was similar to that of the solution used to mix the measles vaccine, the Guardian newspaper reported.
The immunisation programme was suspended following the incident and the health minister of the opposition interim government resigned. Officials said the investigation was continuing to find out who was responsible, with inquiries now reportedly focused on clinical negligence.
The World Health Organization (WHO) said it had "provided a team of experts to provide assistance in investigating this event" but that it was vital that immunisation efforts resumed in Syria as soon as possible.
The charity Save the Children, which provides aid to northern Syria, meanwhile warned on Thursday that education had become one of the deadliest pursuits for children and teachers, as the country's schools were increasingly being damaged and destroyed.
Across Syria, at least 3,465 schools have been destroyed or damaged, and many have been occupied for military purposes.
A new report by Save the Children found that from almost 100% enrolment, Syria now had the second worst rate of school attendance in the world, with 2.8 million children out of school. Enrolment in the hardest hit areas such as Aleppo has fallen to just 6%.
Up to half of the children surveyed said they were "rarely" or "never" able to concentrate in class.
Syrian refugee children in neighbouring countries were meanwhile facing disturbing rates of abuse, bullying, corporal punishment and marginalisation, the report said.
"It is absolutely shameful that the obligation to protect schools is not being respected in this conflict, endangering the lives of innocent children and dedicated teachers and forcing millions out of education," said Save the Children's regional director Roger Hearn.