"The concentration of dangerous elements would be so insignificant by the time the river carries it into the sea, that it would be simply undetectable for the fish," said the chief of the Pacific fish industry research institute's department in the city of Khabarovsk, German Novomodny.

"Even in spring, when the benzene now frozen in ice and nitrobenzene which now sinks to the bottom get into the river, this will not poison the sea fauna," Novomodny said.

The spill was caused by an explosion at a chemical factory in China on November 13 that spewed 100 tonnes of benzene, a known carcinogen, into the Songhua river, a tributary of the Amur which runs along the Russia-Chinese border before entering Russian territory above Khabarovsk.

In Khabarovsk itself the 600,000 local people will have to bear the effects of the slick for three days as it makes its way downstream but experts insist pollution levels remain within acceptable levels.

The regional office of the emergency situations ministry told AFP that the main slick had started passing through the city early Sunday and would take three days to clear it.

The front edge of the slick reached Khabarovsk on Thursday and the regional emergency situations ministry said Sunday the front was now 50 kilometresmiles) downstream from the city.

The authorities said mobile labs monitoring the river would be re-located down river to Amursk, the next large city the slick would pass.

In spite of the presence of benzene, nitrobenzine and other toxic chemicals the authorities said the slick did not represent a threat to health as benzene concentrations were low.

They were at only a tenth of the level judged dangerous and had actually dropped compared with previous days when an earlier section of the slick had passed through the city.

Much of the benzene that originally entered the river is thought by experts long since to have evaporated.

But nitrobenzene, an oily, colourless or pale yellow liquid with a characteristic smell of bitter almonds whose effects on people range widely from drowsiness to death and which can effect fertility and liver functions, could still be present in high quantities.

The authorities decided Saturday there was no danger to people and the emergency committee handling the crisis was dissolved.

The apparent paradox -- the arrival of the main part of the slick yet a drop in benzene levels -- could be explained by "several factors", according to Nikolai Efimov, programme coordinator of the WWF environmentalist organisation.

"First, when the temperature is below freezing nitrobenzine forms as sediment on the bottom of the river bed, whereas benzene is absorbed by the ice," he said.

Furthermore, tributaries such as the river Ussuri brought in unpolluted waters, lowering the concentration of chemicals. The release of water from Chinese hydroelectric dams had raised the water level by a 1.5 metres (five feet).

"That doesn't mean the toxic products have vanished, just that their concentrations have dropped," Efimov said.

The regional authorities are worried about what might happen in the spring when snow and ice which have trapped the chemicals melt.

It will be necessary to "regularly and permanently survey the state of the ice and the bottom of the river," the office of the local governor Viktor Ishayev said.

The authorities also plan to present China with a bill, estimating the cost of cleaning up the pollution at "more than 200 million rubles" (6.9 million dollars).

The slick is due to reach the next major town on the river, Komsomolsk-na-Amure, and its 400,000 inhabitants on January 5 and then flow into the Okhotsk sea.

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