At a regular news conference on Wednesday, WHO Director-General Tedros Adhanom Ghebreyesus said globally reported cases per week have risen nearly 30 percent over the past two weeks.
He pointed out that the Omicron subvariants BA.4 and BA.5 have become dominant in Europe and America.
He said virus testing has been reduced in many countries, obscuring the true picture of infection.
Tedros said vaccines' protection wanes as the virus evolves, even though they are still effective in preventing serious symptoms and deaths.
The WHO chief called for action at global, national and local levels to address the challenge.
He also called for an additional vaccine shot for people at risk, including seniors and medical workers, and urged people to wear a face mask if they are in a crowded place or a room with poor ventilation.