"We are taking measurements every three hours at many points," said Valentina Zozulina, senior laboratory engineer with the Institute of Tectonics and Geophysics with the Russian Academy of Science, who confirmed that toxic nitrobenzene had been detected in waters in Khabarovsk.

She said however that the highest concentrations detected so far in the wider Khabarovsk region were well below acceptable limits and were substantially lower in the waters within the limits of the city itself, which is home to around 600,000 people.

Maximum toxic concentrations were expected to flow into the city from Friday and the entire slick, which has stretched to 190 kilometers (120 miles) in length, will take up to four days to flow through Khabarovsk.

Scientists predicted however that even the highest concentrations would stay below maximum tolerance levels. Officials said new charcoal filters were capable of cleaning the river water and Khabarovsk city did not plan to shut off public water supplies.

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

Experts however explained that much of the benzene that entered the river then had long since evaporated but that concentrations of nitrobenzene, a benzene derivative that can also pose dangers to human health, were still in the river.

Effects on people from excessive exposure to nitrobenzene range widely from drowsiness to death.

A spokesman for the Russian emergency situations ministry told AFP that the Amur waters tested above Khabarovsk showed a higher-than-average presence of nitrobenzene as well as benzene and other potentially harmful chemicals.

Zozulina told reporters that the levels of nitrobenzene registered so far "are absolutely safe for people."

The governor of the Khabarovsk region, Viktor Ishayev, cautioned however that just because the water was technically safe it was not necessarily good to drink, urging residents to use only bottled water for the time being.

"I cannot be certain that the water is of good quality," Ishayev said. "'Safe' does not mean 'good'." He did not elaborate. However he repeated warnings to locals not to eat fish from the Amur river.

"You cannot eat fish from the Amur, and we do not yet know for how long this will remain in effect," Ishayev said.

People in Khabarovsk seemed to be going about about their business as usual on Thursday, with little expression of public anger at China over the contamination, which officials last month described as a major ecological disaster.

Ishayev said the poisoning highlighted growing problems with the ecology in the Amur river region.

"China is developing quickly, and who knows what lies ahead for the Amur and its channels," he said. "We can handle water purification for our cities, but the problem of pollution of the river needs to be solved."

While the Chinese and Russian governments have played down any notion of hard feelings over the spill, it has nonetheless put a strain on relations at a more local level, with Khabarovsk officials recalling at every opportunity that they are demanding financial compensation from China.

The crisis has also however produced incidents of extraordinary cross-border cooperation between the Chinese and the Russians.

One of the most striking examples of this occurred earlier this week when thousands of Chinese workers joined forces with Russian military helicopters, trucks and other heavy equipment in a rush to complete construction of a makeshift dam blocking one of the Amur channels to stem the flow of the poison.

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