At least 34 children have died in rebel-held Syria after being injected with contaminated measles vaccines, the rebel government said on Tuesday, warning that the deaths might be caused by saboteurs linked to the Assad regime.

The death toll is expected to rise, with many children from eight towns in Idlib province, north-west Syria, still in intensive care on Tuesday night.

Parents accused opposition health authorities of failing to store the vaccines properly, and of supplying out-of-date medication. But opposition officials denied accusations of negligence, saying that the vaccines came from Unicef and WHO, via the Turkish government, and that the same batch had successfully vaccinated 60,000 schoolchildren in 30 locations in recent days.

Instead, the opposition health ministry attributed the deaths to foul play, with health minister Adnan Hazouri promising to resign if an investigation upheld allegations of negligence. "Primary investigations point to a limited security breach by vandals likely connected to the regime, which has been attempting to target the medical sector in Free Syria in order to spread chaos," the ministry said in a statement.

"The symptoms don't just indicate spoiled vaccines – it suggests they've been contaminated," Bashar Kayal, another health official, told Radio Hawa Smart, a Syrian-focused station based across the border in Gaziantep, Turkey.

Blood samples have been sent to Turkey for analysis, and the vaccination programme suspended. These moves have done little to convince parents in Idlib province, who fear many more children might be affected.

But the head of Idlib's medical department, Monther Khalil, claimed the worst was over. "The department assures all parents who have had their children vaccinated that the vaccine is completely fine and there is no risk to children who have already been injected," Khalil told Radio Hawa Smart, a Syrian-focused station based across the border in Gaziantep, Turkey.

"We have already vaccinated 60,000 children against measles and there has been no previous problem. The same crews also previously carried out a polio campaign, where they vaccinated 252,000 children across seven rounds, and there were no abnormal complications."

Idlib province is held by moderate rebel groups opposed to both Syria's dictator, Bashar al-Assad, and the extremist Islamic State (Isis). The area was the subject of a fierce military campaign earlier this year, which saw the rebels regain control.

The Syrian civil war has already claimed the lives of over 191,000 Syrians, and created more than three million refugees. The US is currently rallying global support.