Six Chinese companies have been fined $26 million for discharging tens of thousands of tonnes of waste chemicals into rivers, state media said, the biggest such penalty ever in China.

The firms in Taizhou in the eastern province of Jiangsu were sued by a local environment protection organisation and were found to have dumped 25,000 tonnes of waste hydrochloric acid into two rivers, the Xinhua news agency reported.

A court in the city ordered the companies to pay 160 million yuan ($26 million) in fines earlier this year, the highest ever penalty in Chinese environmental public interest litigation, and a higher court upheld the punishment Tuesday, it said.

In August, 14 people involved in the case were sentenced by another court to prison terms of two to five years for causing environmental pollution, it added.

The previous heaviest penalty for polluting the environment was a fine of 37.14 million yuan meted out to three defendants in neighbouring Shandong province, also for contaminating rivers, according to an earlier report by the Southern Weekly newspaper.

Three decades of rapid and unfettered industrial expansion have taken a heavy toll on China's environment, and Communist leaders have been concerned by an increasing number of angry protests over the issue.

Recent studies have shown that roughly two-thirds of China's soil is estimated to be polluted, and that 60 percent of underground water is too contaminated to drink. Meanwhile residents of cities such as Beijing and Shanghai are regularly confronted with hazardous smog levels.

Premier Li Keqiang announced in March that the country was "declaring war" against pollution, and a series of measures have been announced, but questions remain over enforcement.

China in April amended its environmental protection law -- the first such move in 25 years -- imposing tougher penalties and pledging that violators will be "named and shamed".

But holding polluters legally accountable has proved difficult in a country where local governments are often focused on driving growth.