Authorities have cut water supplies to 3.8 million residents in the Chinese city of Harbin to prevent widespread poisoning from the carcinogenic chemical, which is expected to flow downstream into Russia in two weeks.

WWF said in a statement that it was "highly concerned that a toxic spill in China will have an adverse impact on the region's people and environment, as well as spreading through the river ecosystem to Russia".

Freshwater resources are under pressure in China with rapid economic development and the huge population growth in the world's most populous nation, especially along its major rivers, according to WWF.

The global environmental group estimated that 70 percent of the rivers in China are polluted by untreated sewage and industrial waste.

"We need much stronger national and international laws to ensure that hazardous and highly toxic substances, like those released in the explosion, are either not produced or are severely restricted," said Cliff Curtis, head of WWF's Global Toxics Programme.

The chemical spill took place in a high priority conservation region for the WWF which is home to tigers, leopards, bears and "some of the world's most distinctive temperate forests".

"The region's precious natural resources must be protected as a result of this spill," said Dr Li Lifeng, Director of WWF China's Freshwater Programme. "We need to work together to ensure a healthy ecosystem and ecological security."

Chinese environmental authorities said the slick released by an explosion at a petrochemical factory on November 13 was weakening and its impact was expected to decrease.

The 1,897-kilometre (1,176-mile) Songhua flows into the Heilong river at the border with Russia, and supplies the Russian city of Khabarovsk with water.

Russian authorities have already expressed concern about the benzene slick, saying water supplies for a total of 1.5 million residents in and around Khabarovsk may be contaminated.

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