Russian environmental officials have detected an increase in the level of cancer-causing benzene in the Amur River, but they said it is not clear if it is from the November 13 Chinese chemical spill.

Officials in Russia's far east said it could take several more days before the worst of the spill reaches Russian territory.

Also Wednesday, scientists with the non-profit World Wildlife Fund rejected claims that the toxic slick poses less of a threat as it becomes diluted. They said the full extent of the damage may not be understood until well after the spring thaw, when melting ice frees trapped chemicals.

An explosion at a Chinese factory two weeks ago released some 100 tons of cancer-causing agents. They are now flowing down China's Songhua River towards Russia, forcing cities to shut down water systems over contamination fears.

Some information for this story provided by AP and AFP.