More than 110 tons of dead fish have been removed from the Fu River in Hubei province in central China since villagers began noticing the problem Monday, state-run Xinhua News Agency reported.
The World Health Organization took samples from the water and found 196 milligrams per liter of ammonia -- far exceeding the natural 2 milligrams per liter normally found in freshwater. Drinking water has about 0.2 milligrams per liter of the chemical.
The New York Times said Chinese officials have identified pollution from a factory owned by Hubei Shuanghuan Science and Technology Co., as the source of the ammonia. The plant makes sodium carbonate for use in glass and ammonium nitrate for fertilizer. The company's stock was pulled from China's Shenzen stock exchange prior to trading Wednesday.
Chinese officials stressed the Fu River is not a source of drinking water though Xinhua said local residents frequently use it for fishing.
The Fu River does, however, flow into the Yangtze, China's longest river, the watershed for which is a source of drinking water for 40 percent of the nation's population.
Chinese environmental officials including Ma Jun, director of the Institute of Public and Environmental Affairs, a Chinese nongovernmental organization that tracks air and water pollution there, said keeping the Yangtze clean is a major concern for the country.
The company responsible for the Fu River pollution had been cited for times for environmental lapses since 2008, Ma said.
"Each time it was ordered to be corrected, but this demonstrates that enforcement is way too weak and the cost of violations way too low," Ma said.