China has blamed toxic contamination from a petrochemical plant accident for a decision to cut off water to millions of people in a frozen northern city, saying that pollution of a major river was 100 times above safety levels.

Frightened residents of Harbin jammed airports and stations today, trying to escape the city where officials cut off the water for at least four days starting from midnight on Tuesday.

Supermarket shelves in the city have been swept bare of bottled water and soft drinks by panic buying as citizens stocked up for the four-day shortage. One factory manager said: "Everyone wants to leave Harbin and it is very difficult to buy tickets, just like during the Lunar New Year." Schools cancelled classes until next week.

The Government finally gave citizens the reason for the shutdown of the water supply to 3.8 million of the nine million residents of municipality in a move to calm wild rumours that had fuelled the panic buying.

The Harbin Communist Party secretary, Du Yuxin, said: "The recent explosion of two benzene processing towers at PetroChina’s Jilin Petroleum and Chemical Company could end up polluting waters in the Songhua river."

Confusion remained over when water supplies would restart after officials withdrew an earlier notice announcing a four-day stoppage. The State Environmental Protection Administration said that water in some parts of the Songhua river had recorded benzene contamination more than 100 times above safety levels.

Residents reported seeing dead fish washed up on the banks. The river flows north from Jilin province, through Harbin - the capital of Heilongjiang province - and into Russia. The pollution had decreased and contamination was estimated at about 29 times safety levels as the polluted water approached Harbin.

The Foreign Ministry said that China notified Russia of the problem on Tuesday and the two countries were in frequent contact to keep abreast of the flow of the contamination.

Officials estimated that the worst of the pollution would flow through Harbin in the early hours of Thursday morning. A total of 15 hospitals in the city have been put on alert to handle contamination cases.

Benzene, an industrial solvent and component of petrol, is a toxic chemical 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. It is listed as a carcinogen although it is safe in very small doses.

More than 16,000 tonnes of bottled water was being transported to Harbin from nearby cities. Residents had filled bathtubs and buckets and many said they had sufficient supplies to manage for four days. However, several voiced anxiety that they could run out of water if the stoppage lasted for more than four days.

One restaurant owner said: "Today is only the first day and I have no confidence that the water supply will restart in four days. The government didn’t tell us the real reason so there are lots of rumours and fears. I hope the government will just tell us what they know."

Companies that supply steam heat to buildings have been ordered to ensure they have adequate water supplies from wells to ensure that heat is not interrupted. Harbin is one of the coldest places in China, with overnight temperatures this week of -12C (10F).

It is best known for its winter ice lantern festival, when giant slabs of ice cut from the Songhua are used to construct copies of famous buildings and artworks in public parks. Much of the surface of the river has already frozen over.

The ordeal of Harbin’s residents highlighted the precarious state of China’s water supplies. The country’s 1.3 billion people and the factories and farms of its booming economy compete for scarce supplies, while the Government says all of China’s major rivers are dangerously polluted. Due to its vast population, China ranks among countries with the smallest water supplies per person.

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