Twenty Iranian schoolgirls were hospitalised on Tuesday after the latest in a mysterious wave of poisoning attacks that have sickened thousands of pupils, state media reported.

The girls were treated after suffering shortness of breath, in Tabriz, capital of the northwestern province of East Azerbaijan, reported the news agency IRNA.

"Emergency experts were immediately dispatched to the scene after a report that a number of students from one of the girls high schools in Tabriz were in a bad condition," Asghar Jafari, head of the city's emergency service, told IRNA.

The "vital signs... and the general condition of all students is good", he added.

In a wave of cases since late November, more than 5,000 students have suffered fainting, nausea, shortness of breath and other symptoms after reporting "unpleasant" odours, with some needing hospital treatment.

The unexplained attacks in about 230 schools have sparked fear and anger among pupils and their parents.

Schools had resumed classes on Monday following the two-week Nowruz holiday.

In early March, Iran announced more than 100 arrests over the 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.

Schools across 25 out of Iran's 31 provinces have been targeted in the attacks, the latest official tally said.