The head of the World Health Organization on Wednesday said he was "extremely concerned" by the unprecedented wave of COVID-19 cases in China, as the global health body urged China to accelerate vaccinating the most vulnerable.

"WHO is extremely concerned with the deteriorating situation in China, with increasing reports of severe cases," Tedros Adhanom Ghebreyesus told a weekly news conference, appealing for detailed information on severe cases, hospital admissions and intensive care requirements.

"The WHO is supporting China to focus its efforts on vaccinating people at highest risk across the country, and we continue to offer our support for clinical care and protecting its health system", he added.

Since 2020, China has imposed strict health restrictions as part of a so-called "Zero COVID-19" policy.

But the government ended most of those measures without notice in early December amid growing public exasperation and a significant impact on economy. The number of cases has since soared, raising fears of a high mortality rate among the elderly, who are particularly vulnerable.

Chinese authorities said on Tuesday that only those who had directly died of respiratory failure caused by the virus would now be counted under COVID-19 death statistics.

The change in the criteria for recording virus deaths means most are no longer counted, and China said on Wednesday that not a single person had died of COVID-19 the previous day.

WHO emergencies chief, Michael Ryan, stressed on the need for more vaccinations saying, "We have been saying this for weeks that this highly infectious virus was always going to be very hard to stop completely, with just public health and social measures. Most countries have really transitioned to a mixed strategy. Vaccination is the exit strategy in that sense from the impact of a wave of Omicron – the prevalent COVID-19 variant."