It took 67 days from the first reported of Covid-19 to reach 100,000 cases, 11 days for the second 100,000, and just four days for the third 100,000.
But WHO Director General Tedros Adhanom Ghebreyesus said it was still possible to "change the trajectory".
He urged countries to adopt rigorous testing and contact-tracing strategies.
"What matters most is what we do. You can't win a football game by defending. You have to attack as well," he told a joint news conference with Fifa president Gianni Infantino to launch a "kick out coronavirus" campaign featuring footballers.
Dr Tedros said asking people to stay at home and other physical-distancing measures were an important way of slowing down the spread of the virus, but described them as "defensive measures that will not help us to win".
"To win, we need to attack the virus with aggressive and targeted tactics - testing every suspected case, isolating and caring for every confirmed case, and chasing and quarantining every close contact."