The UN agency's chief Tedros Adhanom Ghebreyesus said that on Tuesday, there were "106,000 cases reported to WHO – the most in a single day since the outbreak began" in December.
The WHO was also getting to grips with US President Donald Trump's reform ultimatum, giving the organisation 30 days to overhaul its operations otherwise its biggest contributor would freeze its funding and consider pulling out altogether.
The Geneva-based WHO's coronavirus disease dashboard said that on Tuesday, 106,662 confirmed cases had been reported to the agency from around the world.
The new figures come after states around the world have been dramatically ramping up their testing programmes.
And the pandemic is still unfolding.
"We still have a long way to go in this pandemic," Tedros told a virtual press conference as his agency warned of rising infection figures in poorer countries.
More than 4.9 million cases of the novel coronavirus have been registered in total since the outbreak first emerged in China last December, according to a tally from official sources compiled by AFP.
WHO emergencies director Michael Ryan said the 5 million cases mark would be a "tragic milestone".
Tedros added: "We're very concerned about the rising numbers of cases in low- and middle-income countries."
More than 325,000 people have lost their lives, according to the AFP tally.
WHO studying Trump letter
The WHO's annual gathering of member states agreed Tuesday to an independent probe into the UN agency's coronavirus response amid mounting US criticism over its handling of the pandemic.
Trump made public later Tuesday a letter he sent to Tedros, saying that if the WHO did not commit to "major substantive improvements" within 30 days, he would permanently freeze funding to the organisation and reconsider US membership.
The United States is the biggest contributor to the WHO's budget and has already suspended funding, accusing the organisation of severely mismanaging and covering up the spread of the virus.
Pressed on the ultimatum, Tedros said only: "We have received the letter and we are looking into it."
The WHO agreed that an "impartial, independent and comprehensive evaluation" of "the actions of WHO and their timelines pertaining to the COVID-19 pandemic" should be conducted at the "earliest possible moment".
Asked Wednesday when that might be, Tedros said: "When all the conditions we need are actually met".
Trump on Monday made the surprise announcement that he is taking hydroxychloroquine, an anti-malaria drug that his own government experts say is not suitable for fighting the novel coronavirus.
And Brazil's health ministry recommended Wednesday using chloroquine and hydroxychloroquine to treat even mild cases of COVID-19 – treatments President Jair Bolsonaro has pushed for despite the lack of conclusive evidence of their effectiveness.
The WHO's Ryan stressed: "Hydroxychloroquine nor chloroquine have been, as yet, found to be effective in the treatment of COVID-19 – or in the prophylaxis against coming down with the disease."
The two drugs are among a handful involved in WHO-coordinated clinical trials to find effective treatments for the disease. Some 3,000 patients are taking part in the trials in 320 hospitals across 17 countries.
"As WHO, we would advise that for COVID-19, that these drugs be reserved for use within such trials," said Ryan.