Iran's supreme leader said on Monday that the poisoning schoolgirls is an "unforgivable" crime that should be punished by death if deliberate, state TV reported, amid public anger over a wave of suspected attacks in schools.

Over 1,000 girls have fallen ill after being poisoned since November, according to state media and officials, with some politicians blaming religious groups opposed to girls' education.

The poisonings have come at a critical time for Iran's clerical rulers after months of protests since the death of a young woman held by police for flouting hijab rules.

"Authorities should seriously pursue the issue of students' poisoning," Ayatollah Ali Khamenei was quoted as saying by state TV. "If it is proven deliberate, those perpetrators of this unforgivable crime should be sentenced to capital punishment."