LONDON -- The head of the World Health Organization said on Tuesday China's zero-tolerance COVID-19 policy is not sustainable given what is now known of the virus, in rare public comments by the UN agency on a government's handling of the pandemic.

"We don't think that it is sustainable considering the behaviour of the virus and what we now anticipate in the future," WHO Director-General Tedros Adhanom Ghebreyesus told a media briefing.

"We have discussed this issue with Chinese experts. And we indicated that the approach will not be sustainable... I think a shift would be very important."

He said increased knowledge about the virus and better tools to combat it also suggested it was time for a change of strategy.

Speaking after Tedros, WHO emergencies director Mike Ryan said the impact of a "zero-COVID" policy on human rights also needs to be taken into consideration.

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

He also noted that China has registered 15,000 deaths since the virus first emerged in the city of Wuhan in late 2019 - a relatively low number compared with nearly 1 million in the United States, more than 664,000 in Brazil and over 524,000 in India. 

With that in mind, it is understandable, Ryan said, that the world's most populous country would want to take tough measures to curb coronavirus contagion.

Still, China's zero-COVID policy has drawn criticism ranging from scientists to its own citizens, leading to a cycle of lockdowns of many millions of people, anguish and anger. Most other nations that shared its approach initially have now at least begun a transition to strategies to live with the virus.

The continued outbreaks also underscore how difficult it is to stop the spread of the highly transmissible Omicron variant. 

Under zero-COVID, authorities lock down large population areas to stamp out viral spread in response to any coronavirus outbreak, even if just a small number of people test positive.

Shanghai's measures have been particularly strict, with residents allowed out of compounds only for exceptional reasons, such as a medical emergency. Many are not even allowed out of their front doors to mingle with neighbours.

Its quarantine policy has also been criticized for separating children from parents and putting asymptomatic cases among those with symptoms.

(Reporting by Jennifer Rigby and Josephine Mason in London; Editing by Mark Heinrich and Lisa Shumaker)