Iran’s supreme leader said on Monday that poisoning of Iranian schoolgirls in recent months is an “unforgivable” crime amid spread of suspected poisoning across the country with hundreds of girls hospitalized.

“Authorities should seriously pursue the issue of students’ poisoning. This is an unforgivable crime… the perpetrators of this crime should be severely punished,” Ali Khamenei was quoted as saying by state media.

According to Reuters, the spreading attacks that started in November in Qom in central Iran have spread to at least 25 of Iran’s 31 provinces, prompting some parents to take their children out of school and hold protests against the establishment.

Over a thousand Iranian girls in different schools have suffered “mild poison” attacks since November, according to state media and officials.

A number of politicians said they could have been targeted by religious groups opposed to girls’ education.