The figure could be even higher without concerted action to curb the pandemic, Dr Mike Ryan, head of the UN agency's emergencies program, told a briefing on Friday.
The number of deaths about nine months since the novel coronavirus was discovered in China is approaching one million. "Unless we do it all, the number you speak about is not only imaginable, but unfortunately and sadly, very likely."
The start of a second surge of coronavirus infections has been seen in many countries in the northern hemisphere as winter approaches.
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.
"Are we prepared to do what it takes to avoid that number?" Dr Ryan asked, calling on governments to do everything to control Covid-19.
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