Rozali Ismail, the executive chair of Syabas, a private water treatment and supply company for the state, said levels of ammonia in the river providing water to the areas were "very dangerous."

He said the run-off occurred after heavy rains and the collapse of an earthen wall separating the dump site from a river tributary.

"Someone must be responsible and make sure the rivers are clean," he was quoted as saying in the New Straits Times.

The contamination was the second case in western Selangor state in recent months.

In February, residents in Kuala Lumpur and surrounding areas were treated to foul smelling tap water after a river supply was polluted by leakage from a landfill and pollution from activities along the river's banks.

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