A muscle relaxant appears to have been mixed mistakenly with measles vaccine, killing 15 children in Syria this week, the World Health Organization (WHO) said on Friday, calling it the biggest such tragedy in memory.

The WHO said it could not completely rule out sabotage, so the measles inoculation campaign remained suspended until the investigation was finished.

Fifteen children died after being vaccinated against measles in northern Syria, aid workers said on Wednesday, a tragedy likely to damage trust in health services in opposition-held areas.

The manufacturer, who has not been identified, shipped the vaccine in powder form with a diluent to a hub in Syria where it was stored and then sent to Deir al-Zor and Idlib provinces for the campaign to vaccinate tens of thousands of children that began on Monday, WHO spokesman Christian Lindmeier said.

"In the hub apparently, (from) what we know so far, the diluent was kept ... together in the same refrigerator with a muscle relaxant. The relaxant is called Atracurium. This got mixed in some cases instead of the diluent with the vaccine powder," Lindmeier told a news briefing in Geneva.

Pointing to human error, he said: "So the ones who packed it obviously put the wrong ampoules with the vaccine powder into the package. Then it gets shipped in the vaccine carriers to the facility, there it gets unpacked, mixed and then it has to be used within 6 hours.

"So both at the packing and at the unpacking there had to be gross negligence," Lindmeier said.

The muscle relaxant, usually administered as an anesthetic for surgery, works according to weight, so all the children who died were under the age of two, he said. Older children survived after vomiting, diarrhea and anaphylactic shock.

Lindmeier said: "It seems very clear that it was not the manufacturer's fault, not that the vaccine is contaminated, but it's a fault on the ground, again not established whether it's human error or deliberate, but the fault lies on the ground as per indications."

It was not clear who was in charge of the refrigerator and investigations continue, he said. The WHO and UNICEF supported the campaign but were not directly involved due to insecurity and the "politically-charged" situation in the area, he added.

"There is still a slight possibility that it's not only human error, but an intent, that has to be cleared up definitely before anything can continue," he said.

More than 50,000 children in the two provinces received the vaccine before the campaign was suspended, he added.

Prior to Syria's civil war, some 99 percent of children were vaccinated against measles, a highly contagious disease that can cause serious complications such as meningitis and pneumonia, becoming much deadlier in difficult conditions.