Iran announced Tuesday it had made the first arrests in a spate of mystery poisonings of schoolgirls that has affected more than 5,000 pupils since late November.

Supreme leader Ayatollah Ali Khamenei had called Monday for the perpetrators of the "unforgivable crime" to be tracked down "without mercy" as public anger mounts.

"Based on the intelligence and research measures of the intelligence agencies, a number of people have been arrested in five provinces and the relevant agencies are conducting a full investigation," deputy interior minister Majid Mirahmadi told state television.

He did not identify those detained or elaborate on any possible motive.

Scores of Iranian schools have been hit by poisonings since late November, with pupils suffering symptoms ranging from shortness of breath to nausea and vertigo after reporting "unpleasant" odours on school premises. Some have been treated in hospital.

"Twenty-five (out of 31) provinces and approximately 230 schools have been affected, and more than 5,000 schoolgirls and boys poisoned," Mohammad-Hassan Asafari, a member of the parliamentary fact-finding committee, told the ISNA news agency on Monday.

"Various tests are being carried out to identify the type and cause of the poisonings. So far, no specific information has been obtained regarding the type of poison used."

The mystery poisonings have triggered a wave of anger and demands for action from the authorities.

"Fortunately, so far, no toxic or dangerous substances have been found in any of the students transferred to medical centres."

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