With both landmarks expected to be reached within days, the WHO said it was never too late to take action to suppress the COVID-19 crisis that has gripped the planet, AFP reported.
"This week we'll reach 20 million registered cases of COVID-19 and 750,000 deaths," WHO chief Tedros Adhanom Ghebreyesus told a virtual press conference.
"Behind these statistics is a great deal of pain and suffering. Every life lost matters. I know many of you are grieving and that this is a difficult moment for the world.
"But I want to be clear: There are green shoots of hope and... it's never too late to turn the outbreak around."
Tedros gave examples of countries that had successfully clamped down on the spread of the virus, citing New Zealand and Rwanda.
He also praised Britain and France for taking swift recent measures to tackle new spikes, such as imposing lockdowns around clusters in northern England and making masks compulsory in busy outdoor spaces in Paris.
"My message is crystal clear: Suppress, suppress, suppress the virus," he said.
"If we suppress the virus effectively, we can safely open up societies."
The virus has killed at least 731,518 people since the outbreak emerged last December, according to a tally from official sources compiled by AFP at 1100 GMT on Monday.
At least 19,884,260 cases have been registered in 196 countries and territories. Of these, at least 11,879,100 are now considered recovered.
US hits 5m cases
The United States is the worst-hit country with a total of 162,938 deaths from 5,044,864 cases. At least 1,656,864 people have been declared recovered.
After the US, the hardest-hit countries are Brazil with 101,049 deaths from 3,035,422 cases, Mexico with 52,298 from 480,278 cases, the United Kingdom with 46,574 deaths from 310,825 cases, and India with 44,386 deaths from 2,215,074 cases.
The country with the highest number of deaths compared to its population is Belgium with 85 fatalities per 100,000 inhabitants, followed by United Kingdom with 69, Peru 64, Spain 61 and Italy 58.
China – excluding Hong Kong and Macau – has to date declared 84,668 cases (49 new since Sunday), including 4,634 deaths (0 new), and 79,232 recoveries.
Overall Latin America and the Caribbean have 220,067 fatalities from 5,563,965 cases, Europe 213,353 from 3,355,187 infections, the United States and Canada 171,955 deaths from 5,164,268 cases, Asia 72,391 from 3,480,272, Middle East 30,137 from 1,247,825 cases, Africa 23,269 from 1,049,405 cases, and Oceania 346 from 23,344 cases.