"There's about 50 metres left" to build of the projected 200-metre (650-foot) long dike, said Nikolai Yefimov, head of the local branch of the WWF environmental group.

The dam will block a river channel upstream from Khabarovsk in the hope of diverting a 130-kilometre (81-mile) long benzene slick away from the city of 600,000 people.

The Amur River splits into numerous branches above Khabarovsk. Officials hope that by blocking the Penzenskoy channel that the city will be largely untouched by the pollution.

Russian officials believe the slick from an explosion at a PetroChina chemical plant last month will reach the Amur, which forms the border between China and Russia above Khabarovsk, on December 15 and then approach the city towards the end of December.

Due to increasingly icy conditions the speed of the slick flowing down the Songhua River has slowed to around 20 kilometres (12 miles) a day.

City authorities have been stocking up on bottled water and making other provisions in preparation to shut off mains water supplies.

"Judging by laboratory tests that are being carried out by specialists every four hours, the pollution of the Amur near population centres will not be too high," said Boris Voronov, director of a local environmental institute, playing down fears of an environmental catastrophe.

The concentration of carcinogenic benzene and nitrobenzene from the slick will be minimal and likely evaporate when the ice melts in spring, Voronov said.

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