The number of new coronavirus infections rose last week for the first time in seven weeks, the World Health Organisation said on Monday.
"We need to have a stern warning for all of us: that this virus will rebound if we let it," Maria Van Kerkhove, the WHO technical lead for Covid-19, told a briefing. "And we cannot let it."
WHO Director General Tedros Adhanom Ghebreyesus said the rise in cases was "disappointing but not surprising" and urged countries not to relax measures to fight the disease.
It is too early for countries to rely solely on vaccination programmes and abandon other measures, he said. "If countries rely solely on vaccines, they are making a mistake. Basic public health measures remain the foundation of the response."
Dr Tedros said that Ghana and the Ivory Coast on Monday became the first countries to begin vaccinating people with doses supplied by Covax, the international programme to provide vaccines for poor and middle-income states.
But he also criticised rich countries for hoarding vaccine doses, saying that it was in everyone's interest for vulnerable people to be protected around the world.
"It's regrettable that some countries continue to prioritise vaccinating younger, healthier adults at lower risk of diseases in their own populations, ahead of health workers and older people elsewhere," Dr Tedros said.
Mike Ryan, the WHO's top emergency expert, said the global fight against the coronavirus was in a better state now than it was 10 weeks ago before vaccine distribution had begun. But it was too early to say the virus was coming under control.
"The issue is of us being in control of the virus and the virus being in control of us. And right now the virus is very much in control."