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.

- 'We're definitely afraid' -

One resident who lives near an affected stretch of the Lei River, in Leiyang, told AFP on Tuesday that her family has continued to use water from the tap despite health concerns.

"We're definitely afraid", 37-year-old Xiaoshu, who asked to be identified by a nickname, told AFP over the phone.

"It's fine for us adults, but what if children or the elderly drink it? What if they get sick?"

The mother of two said she had heard about the contamination online through articles shared in group chats and videos posted to Douyin, China's version of TikTok.

"Everybody's been talking about it", she said, but admitted that local authorities never directly notified her of any water contamination.

Despite worrying about the water's safety, Xiaoshu, who sells fishing equipment, said she has bought some bottled water but can't afford to turn off the tap.

"Our family's economic situation is what it is", she said.

"Wealthy people can use bottled water, but for regular folks like us, we can only do that for the water we drink and use tap water for everything else."

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.

Some on Tuesday took to social media to lament the delayed government response.

"It's truly terrifying, this serious of a situation covered up for this long, and it's even scarier that people may not be aware of the severity of the situation", one user commenting under a post about the contamination wrote.

"If we must die let's die together", read another comment.