The UN and Germany called for investigations on Friday into reports of hundreds of schoolgirls being mysteriously poisoned in Iran.
Iran's President Ebrahim Raisi meanwhile blamed the spate of hospital admissions on unnamed enemies of the country.
The apparent breathing problems have led to fears of a deliberate campaign to shut down girls' education.
There have been reports of 800 or more girls being poisoned in at least 58 schools in 10 provinces across Iran.
"The reports of poisoned schoolgirls in Iran are shocking," Germany's Foreign Minister Annalena Baerbock said on Friday.
"Girls must be able to go to school without fear. That is nothing less than their human right. All cases must be thoroughly investigated."
The UN's human rights office is "very concerned about these allegations that girls are being deliberately targeted under what appear to be mysterious circumstances", spokeswoman Ravina Shamdasani said.
She said the findings of a government investigation should be made public and any perpetrators brought to justice.
The World Health Organisation said it was in contact with Iranian authorities and doctors to understand what was happening.
Mr Raisi, addressing a crowd in southern Iran in a speech carried live on state television, said "the enemy" was trying to "instil fear and security among parents and students".
"This is a security project to cause chaos in the country," he said.
It comes against the backdrop of months-long women's rights protests in Iran, with the poisonings adding to public anger.
Relations between Germany and Iran have been particularly strained following the unrest that erupted over the death of 22-year-old Mahsa Amini.
Ms Baerbock has been at the forefront of western voices condemning Tehran's crackdown and its executions of anti-government protesters.
Iran this week expelled two German diplomats in a tit-for-tat move after Berlin took similar action in response to a death sentence handed to a German-Iranian national, Jamshid Sharmahd.
