"The pandemic is still accelerating," WHO's director general Tedros Adhanom Ghebreyesus told the virtual health forum organised by Dubai in the United Arab Emirates.
"We know that the pandemic is much more than a health crisis, it is an economic crisis, a social crisis and in many countries a political crisis."
Dr Tedros said the greatest threat facing the world was not the virus itself, which has now killed over 465,000 people and infected nine million, but "the lack of global solidarity and global leadership".
"We cannot defeat this pandemic with a divided world," he said. "The politicisation of the pandemic has exacerbated it."
Brazilian President Jair Bolsonaro has repeatedly played down the threat, comparing the virus to a "little flu" and arguing the economic impact of shutdowns is worse than the virus itself.
Brazil is the second worst-affected country behind the United States, another nation where political infighting has prevented a unified policy.
Mexico, Peru and Chile are also coping with severe crises - Mexico City being forced to delay plans for a broad reopening of the economy as the country's death toll raced past 20,000.
With a vaccine still far from being developed, the WHO has now called for a rapid increase in production of the steroid dexamethasone, which has been shown to have life-saving potential for critically ill patients.
In Europe, the feelgood factor continues as countries ease their lockdown restrictions.
Thousands of French danced and partied well into Monday for an annual music festival, in the first big blowout since the lockdown.
Revellers packed the streets of Paris, most shunning masks and social distancing, to enjoy concerts in cafes and on street corners.
Although there were none of the usual extravaganzas, many felt the authorities were too lax.
"This is not what a gradual end to the lockdown looks like," said Dr Gilbert Deray.
"I understand that the Festival of Music is something of a liberation, but did we really have to have it this year?"
Swimming pools and cinemas also reopened on Monday while children up to the age of 15 returned to school, attendance once again becoming compulsory.
But illustrating the persisting risks, Portugal Prime Minister Antonio Costa said restrictions on gatherings of more than 10 people would be reimposed and cafes and shops ordered to close at 8:00 pm in the capital region.
"These are fire outbreaks and as with all fires, they must be responded to with the necessary means to prevent their spread," State Secretary of Health Antonio Lacerda Sales said.
Melbourne travel warning
Australians were also warned on Monday to avoid travelling to Melbourne, as the country's second-biggest city tightened restrictions over fears of an upsurge in cases.
Victoria state has recorded more than 110 cases in the past week - many of them in Melbourne - prompting leaders of other regions to warn against visiting the city's six designated virus "hot spots".
China, Germany and Japan are also battling new outbreaks with some reintroducing containment measures.
The spike in infections increased nervousness in global markets, which mostly fell on Monday.
After enjoying a broadly positive week, traders turned cautious on news of a worrying jump in fresh cases in several US states including California, Texas and Florida.
German airline group Lufthansa, meanwhile, says it has backup plans ready in case shareholders reject a nine-billion-euro ($10.1 billion) pandemic rescue plan agreed with the state.
Like rival airlines, Lufthansa was plunged into crisis after efforts to contain the coronavirus brought air travel to a near standstill for several months this year.
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