Television pictures showed shops unloading bottled water supplies while scientists pushed aside lumps of ice to test the Amur river, which is fed by the Songhua -- Sungari in Russian.

Russia's environmental watchdog said the spill could reach the first Russian settlements in the next two to three days, while the Emergencies Ministry said it could start affecting the major city of Khabarovsk by Dec. 10-12.

"Our main task is to make sure people have water," said Russia's chief state epidemiologist, Gennady Onishchenko, adding that more than one million people could be affected.

"Will we have to turn off the water supply? I cannot yet say with complete certainty," he told reporters.

China's city of Harbin turned its taps on again on Sunday after the spill left millions without water for five days, with an 80-km (50-mile) slick still flowing beyond the city.

Although officials say the slick should be less toxic by the time it crosses into Russia, Onishchenko warned that the dangerous compounds would have been diluted faster if the river was in full spate rather than half-frozen.

The crisis has raised wider questions about the costs of China's breakneck economic boom. Around 70 percent of its rivers are contaminated.

Onishchenko, who is also head of Russia's consumer rights watchdog, said Moscow needed to face up to the likelihood of similar spills happening in the future.

"Such a heavily-populated territory as China by definition cannot give us clean water... We need to switch big towns to more secure sources of water," he said.

Environmentalists have criticised China's response, saying it should have consulted more widely after the spill.