The WHO chief also warned the worst of the coronavirus pandemic has yet to come on Monday.
WHO Director General Tedros Adhanom Ghebreyesus told reporters in Geneva "the worst is yet ahead of us" - but did not specify in what ways the crisis could get worse.
His warning followed the United States and some European countries beginning the process of slowly reopening and emerging from various lockdowns put in place to stop the spread of COVID-19.
"It has a very dangerous combination and this is happening ... like the 1918 flu that killed up to 100 million people," the WHO chief said.
Tedros added because of the technological advances since then "we can prevent that disaster. We can prevent that kind of crisis".
"Trust us. The worst is yet ahead of us. Let's prevent this tragedy. It's a virus that many people still don't understand," said Tedros.
The new coronavirus first surfaced in central China in late 2019. The number of coronavirus positive cases is about 2.5 million worldwide, with the US, Italy and Spain overtaking mainland China in confirmed cases.
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