The chemical slick, which had flowed down China's Songhua river for more than a month, was expected to enter the Amur river at some point during the day, the Xinhua news agency reported.

From there the slick will eventually flow into Russia, according to earlier reports.

Xinhua news agency said the density of the toxic slick, consisting of benzene and nitrobenzene, had declined sharply,

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 the carcinogens into the Songhua River, one of China's longest waterways and a source of water for millions.

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