At least 15 children younger than two years in age have died in clinics in Idlib and Deir Ezzour provinces north-western Syria, after medics administered an anaesthetic instead of measles vaccinations.

Preliminary investigations on the incident, whose first report suggested the death toll could have been as high as 36, revealed that atracuriam, the anaesthetic mistakenly administered can be fatal to infants.

The error is believed to have occurred because the packaging of the anaesthetic drug was similar to that of the solution used to mix the measles vaccination.

Syrian opposition officials initially said the children died because the vaccines from the UN and the World Health Organization (WHO) could have been contaminated by people close to Syria's President Bashar al-Assad.

However, following the preliminary probe, interim Health Minister Adnan Hazouri resigned, as he had vowed he would step down if investigations upheld clinical negligence.

UNICEF and WHO expressed their shock in a joint statement on Wednesday.

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

"Measles is a particular threat to children who have been displaced from their homes and communities, and who are living in camps or other insanitary conditions."

Last October, the UN announced that "millions of Syrian children" were to be vaccinated against measles, polio, mumps and rubella.

Due to the civil war, at least 200,000 children have not received vaccinations in Syria where, prior to the conflict, the rate of vaccination was of 95%.

Syria's civil war, erupted in 2011, has caused more than 190,000 deaths and 6.5 million displaced people, many of whom are in need of urgent medical care.