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Their parents initially accused the opposition health authorities of not storing the vaccines properly or using out-of-date medication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.