The comments by WHO Director-General Tedros Adhanom Ghebreyesus mark a change in tone just months after he said that the world has never been in a better position to end the pandemic.
"We are much closer to being able to say that the emergency phase of the pandemic is over, but we're not there yet," Tedros said on Friday.
The global health agency estimates that about 90% of the world's population now has some level of immunity to SARS-COV-2 either due to prior infection or vaccination.
"Gaps in testing ... and vaccination are continuing to create the perfect conditions for a new variant of concern to emerge that could cause significant mortality," Tedros said.
COVID-19 infections are at record highs in China and have started to rise in parts of Britain after months of decline.
The WHO urged governments globally to focus on reaching those at risk, such as people over the age of 60 and those with underlying conditions, for vaccination.
Reporting by Bhanvi Satija and Manas Mishra in Bengaluru and Josephine Mason in London; Editing by Devika Syamnath and Shounak Dasgupta. »