An emergency response was launched after detecting "abnormal water quality" in a section of the Lei River, officials in the central city of Chenzhou said on Sunday, without giving further details.

In a rare move, multiple news outlets later reported that authorities knew about elevated levels of thallium, a potentially harmful heavy metal, in the river as early as March 16.

The Chenzhou city government admitted that thallium concentrations were at least 13 times higher than normal on that day in a further statement on Tuesday but said the incident "has now been controlled".

Thallium is a tasteless, odourless heavy metal that can damage the nerves, liver, kidneys and other human tissues when consumed in significant quantities.

According to the Southern Metropolis Daily, river monitoring stations first detected the leak in a stretch of river between Chenzhou and nearby Hengyang, a city home to 6.5 million people in Hunan province.

The incident "caused inter-city pollution and threatened the safety of downstream drinking water", the state-backed news outlet said.

It added that officials quietly launched an emergency response the following day, seeking to "eradicate... the danger and uphold social stability".

Authorities did not disclose the incident until Sunday's statement, which said local water was "safe" to drink but did not provide details of the leak or the scope of the government's response.

On Tuesday, the Chenzhou government blamed a local cement factory for the leak, saying rainwater had washed dust containing thallium from a recently dismantled kiln into the river.

It said emergency responders had controlled the pollution source and treated river water to remove the thallium, adding that concentrations had soon returned to normal in downstream areas.

China has cracked down on environmental pollution in recent years, but industrial contamination remains common in many parts of the country.

Media coverage of such incidents has also dried up since President Xi Jinping came to power over a decade ago and tightened the government's grip on the press.