"The water polluted with benzene today reached the Russian border and has flowed into the Amur River," Emergency Situations Minister Sergei Shoigu said in the city of Khabarovsk, near the Chinese border.

This southeastern corner of Russia has been bracing for trouble ever since an explosion last month at a PetroChina chemical factory in China's province of Jilin led to the spill of 100 tonnes of carcinogens into the Songhua River, which flows into the Amur.

Residents of several large Chinese cities saw their water supplies disrupted.

However, initial tests by Russian experts showed the slick, which contains benzene and nitrobenzene, is not as highly concentrated as had been feared and is continuing to dilute, officials from the emergency situations ministry said.

Chinese media also reported that the slick had significantly diluted ahead of entering the Amur, which forms part of the Russian-Chinese border.

Shoigu said the authorities were prepared to ensure safe drinking supplies all along the Amur, including in Khabarovsk, which has a population of 600,000.

"Water reserves and carbon (filters) have been set up. Artesian wells have been reopened," he was quoted as saying in a statement after a meeting of local emergency situations ministry officials. "Khabarovsk's central water system would only be switched off in extreme circumstances."

The first Russian community affected by the spill, which is flowing at a rate of about 30 kilometres a day, was expected to be the village of Nizhne-Leninskoye on Friday, with Khabarovsk being hit on Wednesday.

However, environmental experts say that while Russia may escape serious danger from the immediate arrival of the spill, there may be greater problems in spring when the ice currently blocking much of the waterway begins to melt.

It is feared that Russia faces an environmental time bomb as poisonous substances trapped in ice are freed up and begin to spread. "Already now we must start preparing for the spring floods and the long-term effects of the accident," Shoigu said.

China has been embarrassed by the accident and the risk posed to its giant neighbour, and Beijing has stressed its desire to minimise any damage.

President Hu Jintao said earlier this month that China considered itself "highly responsible to the two countries and the two peoples."

The country's top environmental agency ordered a nationwide inspection of major factories near rivers to prevent a similar disaster from happening in the future.

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