Beirut (AFP) - Syria's opposition said Thursday that 16 children who died during a measles vaccination campaign in the mostly rebel-held province of Idlib had been given an anaesthesia additive by mistake.

"Reports point to the possibility of human error which led to atracurium being used in place of the solvent which should have been used with the vaccine," said opposition government chief Ahmad Tohme.

Tohme said the maximum dose of atracurium as used in general anaesthesia was 0.5 milligrammes per kilogramme (2.2 pounds) of bodyweight, whereas 5 milligramme doses were used with the children, enough to paralyse a child of less than 10 kilos.

An investigation is being carried out to determine any criminal responsibility and the whole vaccination team has been detained for questioning, he said, quoted on the exiled opposition's website.

The opposition ordered a halt to the vaccination programme on Tuesday, reporting 15 deaths, a day after its launch in Idlib province of northwest Syria.

On Thursday, the Syrian Observatory for Human Rights, a monitoring group, said a 16th child had died. Some 100 other children are believed to have been affected and suffered reactions.

The children started showing symptoms "half an hour after innoculation. They suffered diarrhoea, allergic reactions and breathing difficulties," the opposition's health ministry said.

Syria's war has caused more than half of the population to flee their homes and millions of children are among the displaced, both inside and outside the country.

Medical groups have rushed to head off the spread of measles, mumps, rubella and polio in Syria, where normal medical services have disintegrated because of the civil war, which erupted in 2011.