About 500 people, many of them children, received medical treatment after inhaling the fumes from dumped toxic waste.

More than 100 schools in Malaysia were closed after the dumping of toxic waste into a river caused hundreds of people, many of them children, to fall ill.

A truck is believed to have dumped the waste near the industrial town of Pasir Gudang, southern Johor state, last week, sending hazardous fumes across the area and causing those affected to display symptoms of poisoning such as nausea and vomiting.

About 500 people, many of them students, have received medical treatment after inhaling the fumes, with more than 160 admitted to hospital, according to official news agency Bernama.

It was unclear what type of toxic gas had been released.

Education Minister Maszlee Malik initially ordered the closure of 43 schools in the area on Wednesday, but later announced that figure had more than doubled.

“The ministry of education has decided to close all 111 schools in the Pasir Gudang area immediately,” he said in a statement. “The education ministry is requesting that all parties take precautions.”

Maszlee said in a Facebook post the situation was “getting more critical”.

Three men were arrested earlier this week over the toxic waste dumping. One is expected to be charged in court soon and could face up to five years in jail if found guilty of breaking environmental protection laws.