In a report, experts from the Far East division of environmental group WWF rejected claims by some officials that the chemical slick released by an explosion at a Chinese factory on November 13 would pose less of a threat as it became diluted.

Highly toxic metals such as mercury and cadmium would sink to the bottom of the Amur River posing a real threat to fish, and from fish could find their way into humans, the report's author, Lyubov Kondrateva, an expert at the Russian Academy of Sciences, said.

The true scale of the threat might only become clear when the ice on the Amur breaks in the spring, Kondrateva said.

"The toxic waste may accumulate in various organisms and thus enter the human body, causing a real health threat. Once the ice breaks, all this toxic poison could get into the Amur delta and then the sea ... there is a real ecological risk" of the waste getting into the Okhotsk Sea, off Russia's east coast, and the Sea of Japan, she said.

"If that happens, the problem of waste in the border rivers would not concern only Russia and China," Kondrateva added.

Russian authorities have stepped up moves to secure water supplies in the far east of the country, shipping in 20 tonnes of charcoal for use in water purification and increasing stocks of clean water in bottles and cisterns, particularly in the city of Khabarovsk, home to some 600,000 people.

In Moscow on Wednesday the deputy head of the Natural Resource Management Agency, Oleg Mitvol, said that tests to determine whether the slick had already entered Russia had been inconclusive.

At one point west of the city of Khabarovsk, excessive levels of benzene, one of the main components of the slick, had been found, Mitvol said.

"We carried out analyses and found levels of benzene over the permitted limit... I suppose that it is coming from China," Mitvol said.

However Russia's Emergency Situations Ministry, quoted by the RIA-Novosti news agency, denied that pollution from the explosion in China had reached Russian territory.

The toxic slick was due to reach Nizhneleninsk, in Russia's Autonomous Jewish Province, on December 6-8, followed by Khabarovsk on December 9-11, the ministry said.

The blast at a PetroChina plant released 100 tonnes of chemicals, made up mainly of the carcinogen benzene and nitrobenzene, into the Songhua River, a tributary of the Amur.

Harbin, the capital of China's Heilongjiang province, suffered a five-day water shutdown as a result.

Around 50 tonnes of chemicals are believed to have passed through Harbin, continuing to threaten Chinese areas downstream as well as parts of Russia.

Chinese officials in Heilongjiang said Wednesday they were sending experts and equipment to Russia to help curb the slick, following a visit by Russian officials.

"We will send experts to Russia to give training lessons but the equipment will go first, and the experts need to go through some procedures," said a spokesman of the Heilongjiang environmental bureau, who identified himself only by his family name Zhang.

"They are all experienced experts in our environmental protection bureau."

China apologized to its giant neighbour Russia over the weekend for the problems the slick was causing.

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