Director General Dr Tedros Adhanom Ghebreyesus said, "too many countries [were] headed in the wrong direction".
Cases were rising where proven measures were not adopted or followed, he added.
The Americas are the current epicentre of the pandemic. The US has seen a rise in cases amid tensions between health experts and President Donald Trump.
The US, the worst affected country, has over 3.3 million confirmed cases and more than 135,000 deaths, according to Johns Hopkins University.
What has the WHO said?
At a briefing in Geneva on Monday, Dr Tedros said "mixed messages from leaders" were undermining public trust in attempts to bring the pandemic under control.
"The virus remains public enemy number one, but the actions of many governments and people do not reflect this," he said.
Dr Tedros said measures such as social distancing, hand washing, and wearing masks in appropriate situations needed to be taken seriously, warning that there would be "no return to the old normal for the foreseeable future".
"If the basics aren't followed, there is only one way this pandemic is going to go," Dr Tedros said, adding: "It's going to get worse and worse and worse."
Dr Mike Ryan, the WHO's emergencies director, said the easing of some lockdown measures in the Americas and the opening up of some areas had led to "intense transmission".
Latin America has confirmed more than 145,000 coronavirus-related deaths, though the number is believed to be higher because of insufficient testing.
Half of the deaths were in Brazil, where President Jair Bolsonaro has opposed strict measures to curb the spread of the virus.
Dr Ryan said closing down large regions would have huge economic consequences, but that local lockdowns in specific places might be necessary to mitigate the spread of the virus.
He urged governments to implement clear and "strong" strategies, adding: "Citizens have to understand, and it has to be easy for them to comply."