China's censors scrambled to wipe out online debate over its zero-Covid strategy on Wednesday after the World Health Organisation (WHO) criticised the country's hardline approach to crushing the virus.

China is the last major economy glued to a zero-Covid policy and enforces some of the most stringent virus controls anywhere in the world.

Those restrictions have trapped most of Shanghai's 25 million people in a lockdown with no clear end date, while Beijing has also gradually corralled many of its residents indoors as it battles its biggest outbreak since the pandemic began.

On Tuesday WHO chief Tedros Adhanom Ghebreyesus urged China to change tack, saying the approach "will not be sustainable" in the face of new fast-spreading variants.

The intervention prompted China's army of internet censors to race to snuff out his comments.

China's censors scrambled to wipe out online debate over its zero-Covid strategy
China's censors scrambled to wipe out online debate over its zero-Covid strategy AFP / Noel Celis

Searches for the hashtags "Tedros" and "who" on the popular Weibo social media platform displayed no results, while users of the WeChat app were unable to share an article posted on an official United Nations account.

A social media hashtag about the WHO's comments, which had been a rallying point for lively online discussion, appeared to have been blocked by mid-morning.

Before they were expunged from the internet, comments had questioned zero-Covid, with one saying "even the WHO's Tedros has now changed his stance".

Another wrote: "Will our government listen to the WHO director general's recommendations?"

Virus controls are causing mounting anger and frustration, especially in Shanghai where residents have raged against seemingly endless lockdowns, spartan quarantine facilities and heavy-handed enforcement.

A worker disinfects the queue area of a swab test collection site in Beijing
A worker disinfects the queue area of a swab test collection site in Beijing AFP / Noel Celis

The city has witnessed repeated protests and violent scuffles with police, rare images which have pinballed across social media before censors can catch up.

The ruling Communist Party says its virus strategy places life before material concerns and has averted the public health crises seen in other nations.

A foreign ministry spokesman brushed off questions about the WHO's comments on Wednesday, saying Beijing's policy "can stand the test of history" and was "scientific and effective."

"We hope that relevant individuals can take an objective and rational view of China's epidemic prevention and control policies ... and refrain from making irresponsible remarks," Zhao Lijian said at a regular press conference.

Abandoning zero-Covid and allowing Omicron to rip across the country could result in 1.6 million deaths, according to a paper published Tuesday in the peer-reviewed journal Nature by researchers at Shanghai's Fudan University.

Health officials have said vaccination rates are low among the elderly and warn rural health facilities risk collapse under an Omicron surge.

On Monday, vice-premier Sun Chunlan reminded disease control officials of the political imperatives attached to zero-Covid.

It is necessary to "create the conditions for the victorious convening of the 20th Party Congress", she said, according to state news agency Xinhua.

The twice-a-decade conclave scheduled for later this year is expected to see President Xi Jinping secure an unprecedented third consecutive term as the leader of the world's number two economy.

Discussing Beijing's zero-Covid strategy on Tuesday, Tedros said WHO experts "don't think that it's sustainable, considering the behaviour of the virus now and what we anticipate in the future".

Hu Xijin, the influential former editor of Chinese state tabloid Global Times, slammed the comments in a message to his 24 million Weibo followers, saying "in the end, the WHO's attitude isn't important".