"After the blast at the chemical plant the monitoring station in Jilin found that benzene went into the river and polluted the water," the Environmental Protection Administration (EPA) said in a statement on its website.

"Benzene levels were 108 times above national safety levels."

The EPA was referring to a November 13 explosion at the PetroChina petrochemical plant near the Songhua river in Jilin province.

Water supplies to Harbin, the capital of neighbouring Heilongjiang province about 380 kilometers (230 miles) downstream from the blast site, were cut off at midnight on Tuesday over pollution fears.

The EPA said the pollutants in the water were expected to hit Harbin around 5:00 am on Thursday, although the contaminants would be diluted.

The polluting material index had dropped to 29 times above national safety levels when the contaminants reached the border of Jilin and Heilongjiang on Sunday, the EPA said.

Benzene is a carcinogen that can be lethal if someone is exposed to high levels, even in short doses, according to the US National Library of Medicine's website.

The EPA admitted that the chemical slick could be extremely dangerous to people who came into contact with it.

"Chemicals like benzene are very harmful to humans so the state environmental protection administration paid big attention to this case and sent experts to Heilongjiang province," it said.

In Harbin, which has a population of nine million, there had been scenes of panic since the weekend when rumours first surfaced that water supplies would be cut off.

The government initially denied that contaminated water in the Songhua was the reason for the stoppage, leaving residents to speculate about a potential imminent earthquake.

State press reports said water was cut off for the city's 3.8 million urban residents.

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