Iran’s supreme leader said on Monday that poisoning schoolgirls is an “unforgivable” crime which 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 suffered poisoning 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.”

The poisonings began in November in the holy Shia city of Qom and spread to 25 of Iran’s 31 provinces, prompting some parents to take children out of school and protest.

Authorities have accused the Islamic Republic’s “enemies” of using the attacks to undermine the clerical establishment.

But suspicions have fallen on hardline groups operating as self-declared guardians of their interpretation of Islam.