China's zero-Covid strategy to defeat the pandemic is not sustainable, the World Health Organization's chief said Tuesday, adding that the WHO had told Beijing so.

Draconian measures have trapped most of Shanghai's 25 million people at home for weeks as China combats the country's worst outbreak since the pandemic began.

Yet the Shanghai lockdown has intensified, causing outrage and rare protest in the last major economy still glued to a zero-Covid policy.

"When we talk about the zero-Covid strategy, we don't think that it's sustainable, considering the behaviour of the virus now and what we anticipate in the future," Tedros Adhanom Ghebreyesus told a press conference.

"We have discussed about this issue with Chinese experts and we indicated that the approach will not be sustainable and considering the behaviour of the virus, I think a shift would be very important."

WHO emergencies director Michael Ryan said it was time to hit the reset button, saying any measures to combat the Covid-19 pandemic should show "due respect to individual and human rights". 

"We need to balance the control measures against the impact on society, the impact they have on the economy, and that's not always an easy calibration," he said.

Maria Van Kerkhove, the WHO's technical lead on Covid-19, said that worldwide, it was impossible to stop all transmission of the virus.

"Our goal, at a global level, is not to find all cases and stop all transmissions. It's really not possible at this present time," she said.

"But what we need to do is drive transmission down because the virus is circulating at such an intense level."