Education Minister Maszlee Malik on Thursday ordered schools within a 3km radius of Kim Kim River in the city of Johor Bahru, on the southern tip of the Malay Peninsula, to be shut immediately.

In a Facebook post, Malik said the situation was getting “increasingly critical’’ and ordered students and staff to stay away from their schools “until the situation improves.’’

According to the Straits Times, the cause of the poisoning is believed to be toxic fumes from chemicals illegally dumped into the river.

A total of 506 people had been affected by toxic fumes, with nine people in intensive care.

Two schools were first ordered shut on March 7, when students and staff suffered breathing difficulties and fainting after inhaling the fumes.

A second wave of methane poisoning hit just hours after the two schools reopened on Monday.

The latest incident occurred on Tuesday, when 260 people were rushed to hospital with breathing difficulties.

Minister for Energy, Science, Technology, Environment and Climate Change Yeo Bee Yin said that one of suspects of the dumping, the owner of an illegal tyre recycling factory in Johor’s Kulai, is expected to be charged. (dpa/NAN)