Dr Tedros Adhanom Ghebreyesus, director general of the WHO, warned the pandemic has still not reached its peak and the virus is not under control "in most of the world”.
His message comes as quarantine rules for people returning to or visiting the UK from certain countries are relaxed from Friday.
The UK Government has published a list of 76 countries and territories from which people arriving into England will no longer need to self-isolate for 14 days.
But Dr Ghebreyesus said the total number of cases of coronavirus worldwide has doubled in the last six weeks.
Speaking at the member state briefing on the Covid-19 pandemic evaluation, he said: "The virus has upended health systems in some of the world’s wealthiest nations, while some countries that have mounted a successful response have been of modest means.
"We know that when countries take a comprehensive approach based on fundamental public health measures – such as find, isolate, test and treat cases, and trace and quarantine contacts – the outbreak can be brought under control.
"But in most of the world the virus is not under control. It is getting worse."
He added: "More than 11.8 million cases of Covid-19 have now been reported to WHO. More than 544,000 lives have been lost.
"And the pandemic is still accelerating. The total number of cases has doubled in the last six weeks."
The relaxation of the quarantine rules has largely been welcomed by political figures and the tourism industry, although Labour has criticised the Government for failing to set up so-called air bridges to other countries.
Shadow transport secretary Jim McMahon said that workers and holidaymakers were paying the price of the Government’s failure to act.
"The fact they have been unable to negotiate air bridges is an indictment of their failure to tackle the crisis at home," he said.
Meanwhile, for people choosing to shun holidays to stay at home in England, swimming pools, gyms and outdoor arts performances were given the green light to reopen in the latest easing of the lockdown.
Culture Secretary Oliver Dowden said outdoor pools will be able to reopen from Saturday to be followed by indoor pools, gyms and other sports facilities from July 25.
Theatres, opera, dance and music will also be able to resume outdoors from the weekend although audience numbers will be restricted and will be subject to social-distancing rules.
Following the opening for hairdressers last week, beauticians, tattooists, spas, tanning salons and other close contact services will be able to welcome back customers from Monday.
In Scotland, Nicola Sturgeon announced a relaxation of the rules allowing people to mix indoors and confirming plans for bars, restaurants and hairdressers to reopen.
The First Minister said it is “undoubtedly a time for cautious hope and optimism” as she moved the country to phase three of its four-phase easing plan.