More than 1,000 schoolgirls across Iran have complained of what appear to be symptoms of poisoning.
The state news agency says the cases started appearing in November. The areas where they are being reported include Tehran and the central city of Qom.
Symptoms include breathing problems, numb limbs and fatigue. Most cases are mild, and believed to be caused by toxic substances.
The agency says more than 1,000 pupils had complained of such problems by the end of last month.
Some parents and guardians have decided to keep their children at home, and calls are growing to investigate the cause.
On Friday, President Ebrahim Raisi blamed Iran's enemies for what he described as a plan to instill fear and insecurity among parents and students.
On Thursday, White House National Security Council spokesperson John Kirby called for a "thorough and complete" investigation.
And on Friday, a spokesperson for the UN human rights office called for transparency.