Dr Mike Ryan, the WHO's emergencies head, said the figure could be higher without concerted international action.
Almost one million people have died with Covid-19 worldwide since the disease first emerged in China late last year.
Virus infections continue to rise, with 32 million cases confirmed globally.
The start of a second surge of coronavirus infections has been seen in many countries in the northern hemisphere as winter approaches.
So far, the US, India and Brazil have confirmed the most cases, recording more than 15 million between them.
But in recent days, there has been a resurgence of infections across Europe, prompting warnings of national lockdowns similar to those imposed at the height of the first wave of the pandemic.
"Overall within that very large region, we are seeing worrying increases of the disease," Dr Ryan said of the marked spike in cases in Europe.
He urged Europeans to ask themselves whether they had done enough to avoid the need for lockdowns - and whether alternatives, such as testing and tracing, quarantines and social distancing, had been implemented.
"Lockdowns are almost a last resort - and to think that we're back in last-resort territory in September, that's a pretty sobering thought," Dr Ryan told reporters at the WHO's headquarters in Geneva.