"The Songhua toxic slick is fully dissolved in the Amur's waters and practically disappeared. Tests downriver show only minimal traces of nitrobenzene, and so we decided it unpractical to continue maintaining large-scale control," the Far East water and weather agency chief Alexander Gavrilov said.

However, experts warned that benzene and nitrobenzene could have sunk to the river bottom and frozen into ice and would cause a fresh influx in spring, a concern that would set Russian scientists to monitor ice, river bottom and fish as early as February.

One hundred tonnes of the carcinogens benzene and nitrobenzene were released into the Songhua after an explosion in November 13 at a PetroChina chemical factory in the northeastern province of Jilin.

The spill caused an 80-kilometer-long (50-mile-long) toxic slick to flow down the Songhua, the source of water for millions including the residents of Harbin city, who had their water supplies cut off for days.

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