China has detained another company official, bringing the total to eight, over a massive river pollution case in the country's south, the government and state media said Thursday.

Industrial waste -- including toxic cadmium -- polluted up to a 300-kilometre (190-mile) section of the Longjiang River in the Guangxi region and threatened drinking water supplies for millions of people.

Police have detained eight executives from two firms, Jinhe Mining Co. and Jinchengjiang Hongquan Lithopone Materials Factory, according to a statement from Hechi city, where the pollution originated.

Authorities were seeking another four people who had fled, the Shanghai Daily newspaper quoted Hechi Mayor He Xinxing as saying. The government said earlier this week that seven people had been taken into custody.

A Hechi city spokeswoman did not answer phone calls on Thursday.

Jinhe Mining was involved in processing cadmium, a carcinogen which can seriously damage the kidneys, bones and respiratory system.

Jinchengjiang Hongquan was producing the metal indium, outside its business scope, and dumping waste directly into the ground, reports said. Indium can cause lung and other organ damage.

Guangxi claims to have brought the pollution, which was first discovered on January 15, under control amid criticism from state media and environmental groups for poor industry supervision.

In Liuzhou city, downstream from the original spill, the local government said cadmium levels in the river remained at 2.6 times the recommended limit on Thursday morning.

Readings for the heavy metal spiked to 80 times the government limit immediately after the incident, but have fallen after authorities dumped chemical neutralisers and opened sluice gates to increase water flow.

Guangxi has launched a region-wide probe of industry, so far inspecting 145 companies and shutting down 11, state media said. Environmental authorities have also ordered 90 waste disposal sites closed.

Many waterways in China have become heavily contaminated with toxic waste from factories and farms, pollution blamed on three decades of rapid economic growth and lax enforcement of environmental protection laws.

Activists say officials in China often turn a blind eye to industrial pollution or even collude with companies, as they seek to boost local economic development.