"The water polluted with benzene today reached the Russian border and has flowed into the Amur river," which forms the border between the two countries, the minister, Sergei Shoigu, said in the city of Khabarovsk, near the Chinese border.

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.

Shoigu said the authorities were prepared to ensure safe drinking supplies, regardless.

"Water reserves and carbon (filters) have been set up. Artesian wells have been reopened," he was quoted as saying in a statement released by the authorities.

Widespread contamination prevention efforts have been under way in China and Russia since an explosion on November 13 at a PetroChina chemical factory in the northeast Chinese province of Jilin.

The accident led to the spillage of 100 tonnes of carcinogens into the Songhua River, one of China's longest waterways and a source of water for millions of people.

Chinese media also reported that the slick had significantly diluted ahead of entering Russia.

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