Medics carrying out a measles vaccination in northern Syria instead administered an anaesthetic to up to 75 infants, killing 15, a preliminary investigation into their deaths has found.

The report says an anaesthetic, atracuriam, which is used as a muscle relaxant in surgery was given to the infants, perhaps mixed with the measles vaccine, which had been delivered by aid agencies conducting a mass vaccination program in opposition parts of northern Syria.

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.

The drug is usually fatal when given to infants in the doses administered, but older children can survive low dosages due to their higher body weights. The fatalities occurred in children between 6 and 18 months.

The children all presented symptoms within half an hour of being given the drug and were suffering from diarrhoea, shock and respiratory failure. Most of the fatalities were dead before they reached a hospital, according to a local activist.

Syrian opposition officials have ordered an urgent probe into the deaths and the health minister in the interim administration resigned on Wednesday. Earlier reports had suggested the toll may have been as high as 34 and that deliberate attempts may have been made to poison the children.

However inquiries are now focusing on clinical negligence and have vowed to prosecute anyone found responsible.

Save the Children, which provides medical aid to northern Syria, but was not involved in administering the vaccine said: "Save the Children is appalled and deeply saddened by the news of the deaths of a number of children and the hospitalisation of many more after receiving vaccinations for measles in northern Syria. Save the Children will help the authorities in any way we can to help find out what has happened."

International aid agencies have greatly restricted access to northern Syria and have been struggling to deal with massive humanitarian needs caused by a civil war that has led to close to half of Syria's population being displaced. The state-run medical system has all but collapsed in much of the country and people in opposition areas have no access to it.