Kazakhstan's Ministry for Emergency Situations has warned that levels of cyanide in the Sekisovka River in East Kazakhstan Province are 516 times higher than acceptable levels.
The ministry said testing was conducted on October 31 after an unusual number of dead fish were found.
Some the fish were sent for testing and confirmed high amounts of cyanide.
Local residents blamed a nearby gold-processing plant for dumping the cyanide, used to clean gold, into the river.
Schools in the area have been closed and water is being brought in for residents of the affected region.
compiled from local and international agency reports