The European Parliament on Thursday demanded a UN investigation into the poisoning of thousands of school girls in Iran, accusing Tehran of trying to suppress reports of the incidents.

In a wave of cases since late November, students suffered fainting, nausea, shortness of breath and other symptoms after reporting "unpleasant" odours, with some needing hospital treatment.

EU legislators overwhelmingly approved a non-binding resolution urging the UN Human Rights Council to launch a probe into the incidents and demanding "those responsible to be held accountable".

"The resolution condemns the regime's months-long failure to act on, as well as its deliberate suppression of, credible reports of systematic toxic attacks against schoolgirls," the motion said.

It called on the Iranian authorities to let an international mission established by the United Nations "have full access to information on the situation of human rights in the country".

Last week, Iran announced more than 100 arrests over the unexplained poisonings, charging that the unidentified perpetrators may have links with "hostile" groups.

Supreme leader Ayatollah Ali Khamenei has called for the perpetrators of the "unforgivable crime" to be tracked down "without mercy".

The poisonings started two months into the protests that gripped Iran following the September 16 death of 22-year-old Mahsa Amini, who had been arrested over an alleged violation of women's dress rules.

More than 5,000 pupils have been affected in poisoning cases in about 230 schools across 25 out of Iran's 31 provinces, the latest official tally said.

The 27-nation EU has imposed multiple rounds of sanctions on Iranian officials accused of overseeing a brutal crackdown on the protests that rocked the country.

But the bloc has so far stopped short of formally labelling Iran's Islamic Revolutionary Guard Corps a terror group, despite calls to do so from Germany and the Netherlands.

The parliament urged EU countries to designate the Revolutionary Guards a "terrorist organisation" and to expand sanctions to include Khamenei, President Ebrahim Raisi and Prosecutor General Mohammad Jafar Montazeri.