BEIJING (REUTERS) - A Shenzhen-listed company identified by authorities as being responsible for a toxic spill that killed thousands of river fish apologised on Friday and said it would punish those responsible.

Hubei Shuanghuan Science and Technology Stock Co., a manufacturer of inorganic chemicals, said it began a week-long production halt on Wednesday and blamed the spill on a sewage release "that exceeded standard levels."

"The company apologises to impacted parties and the vast majority of investors for this incident," it said.

The company said however, that it was not the only one to blame for the pollution affecting a 40-kilometre stretch of the Fu river, which lies near the heavily populated city of Wuhan in China's central Hubei province.

It said its own investigation found other companies in the area had discharged improperly treated sewage and nearby farms failed to control the flow of animal waste into the river.

Hubei Shuanghuan came under fire from the provincial environmental protection agency when workers unearthed the dead fish, saying ammonia levels in the river had risen far beyond acceptable levels, China's state-controlled Xinhua News Agency reported.

Environmental degradation has proven to be one of the hazards of China's rapid economic growth. In January, choking smog enveloped Beijing for more than a week and in March thousands of dead pigs were discovered floating in a river that provides water to Shanghai.

China's new leadership has identified pollution, long a source of public discontent, as one of its top challenges and has vowed to more strictly police industrial companies that have been given free rein to pollute by local governments eager to maintain jobs and revenue.