Chinese media on Wednesday blasted local officials for poor supervision after industrial waste in a southern river threatened the drinking water for millions of people.

Up to a 300-kilometre (190-mile) stretch of the Longjiang River in the Guangxi region could be contaminated by toxic cadmium and other industrial waste dumped by polluting factories.

Authorities claimed late Tuesday they had brought the pollution under control but the China Daily newspaper said that better supervision of industry was needed and the work of local officials left "much to be desired".

"Local authorities need to investigate thoroughly the root cause of the incident. This incident should be a wake-up call to the rest of the country," it said in an editorial.

Seven company executives deemed responsible for the contamination have been detained.

The Guangxi government said efforts to dilute the pollution by adding chemical neutralisers and allowing a greater volume of water had achieved "clear results", according to a statement.

The government would maintain flow of tap water to Liuzhou city, the statement said, following earlier worries authorities would have to stop supply to some of the city's 3.7 million residents.

Pollution would not affect areas beyond Liuzhou, it added.

"After two weeks of clean-up, the pollution is under control," Feng Zhennian, a local environmental official, was quoted by the official Xinhua news agency as saying.

The Liuzhou government said pollution in some areas was still 2.6 times the national standard for cadmium at midday on Wednesday.

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 push forward local economic development at all costs.

Jinhe Mining Co. has been blamed for dumping cadmium, a carcinogen which can seriously damage the kidneys, bones and respiratory system, into the river in a spill that was discovered on January 15.

Another implicated company, Jinchengjiang Hongquan Lithopone Materials Factory, was producing the metal indium and disposing of waste directly into the ground, state media said.

A doctor at Liuzhou's main hospital said there had been no reports of cadmium poisoning yet and denied doctors had been asked to cover up cases, after accusations the facility would not diagnose such sickness.

He said suspected cases would be reported by the hospital to the local disease control centre. "So far no such patients have been reported," the doctor of the Liuzhou People's Hospital, who declined to be named, told AFP.