GUANGZHOU, Jan. 13 (Xinhua) -- Authorities in south China's Guangdong province said Friday that chemicals leaked from an electrochemical plant was responsible for the deaths of a massive amount of fish in a major river.

A large number of fish were found floating in a three-km section of the Xijiang River in the city of Jiangmen Monday morning.

An investigation revealed that the fish were killed by residual chlorine in the river, which led authorities to the Guangyue Electrochemical Company, about 1.5 km upstream from the section where the dead fish were reported, said an official with the city's ocean and fishery bureau.

The fish were suffocated after chlorine damaged their gills, said the official.

The company confessed that about 10 tonnes of sodium hypochlorite had leaked into the river from a tank at its plant, but it shut down its sewage outlet about 45 minutes later, stopping the leak.

The official said that the company would be severely punished as it did not report the incident.

Sodium hypochlorite solution is frequently used as a disinfectant or bleaching agent.

An emergency response was launched Monday for disposal of the dead fish.

Thursday's monitoring results showed that the river was safe as residual chlorine can easily escape from water with the help of sunshine and wind.

Xijiang River is a major tributary of the Pearl River, the third-longest river in China and a pivotal waterway that runs across the economic heartland of Guangdong.