Dr Tedros Adhanom Ghebreyesus, director-general of the WHO, said that some countries had experienced a “resurgence” in cases after reopening their economies, adding that the pandemic is “speeding up”.
He called for greater cooperation between world leaders to help limit transmission of the disease as countries begin reopening their borders following months of lockdown.
“The worst is yet to come. I’m sorry to say that, but with this kind of environment and condition, we fear the worst,” Dr Adhanom Ghebreyesus told a briefing on Monday.
“And that’s why we have to bring our acts together and fight this dangerous virus together. Some countries are now experiencing a resurgence of cases as they start to reopen...”
Dr Adhanom Ghebreyesus’s comments came as the number of global coronavirus cases edged towards 10.5 million – six months on from when the pandemic first broke out in Wuhan, China.
More than 500,000 people have died after contracting the novel virus, according to official figures, with cases surging in the Americas, Russia and India.
Peru and Chile have seen cases surges in recent weeks. Brazil has now reported more than 1.3 million cases, while officials in the US have logged close to 2.6 million infections.
Infections have so far remained relatively low in much of Africa and Asia outside of China, although experts fear cases in the two continents are yet to peak.
“The virus still has a lot of room to move. We all want this to be over,” Dr Adhanom Ghebreyesus added. “We all want to get on with our lives, but the hard reality is this: this is not even close to being over”.
“With 10 million cases now and half a million deaths, unless we address the problems we’ve already identified at WHO, the lack of national unity and lack of global solidarity and the divided world which is actually helping the virus to spread... the worst is yet to come.”
He also said that it is “a lame excuse” for any country to claim that contact tracing “is difficult” after the WHO warned as early as March that testing would be the key to containing the pandemic.
Dr Adhanom Ghebreyesus’s message remained “test, trace, isolate and quarantine”.
A resurgence of infections in Texas and Florida led officials in the states to reimpose some restrictions on businesses earlier this week.
Officials declared a state of emergency in Brazil capital Brasilia on Monday following a spike in cases – just weeks after the nation had begun to resume business.
In the UK meanwhile, health secretary Matt Hancock was on Monday night forced to put Leicester back on lockdown following hundreds of new infections in recent weeks.
Leicester mayor Sir Peter Soulsby has suggested the new lockdown in the city should have been brought in much sooner, as shops there closed on Tuesday while schools must shut from Thursday.
Speaking on the BBC’s Today programme, he said: “The secretary of state announced that he believed there was an outbreak in Leicester the best part of two weeks ago.
“Since then, we’ve been struggling to get information from them (the government) about what data they had, what led them to believe there was a particular problem here, and struggling to get them to keep the level of testing in Leicester.”
But Hancock insisted the government had not been too slow to act in Leicester, but that targeted action had not worked and so a wider lockdown was required.
“People knew 11 days ago because I set it out that there was an outbreak in Leicester, and we were working on it before then,” he said.
“And I’d been talking to the council and working with public health experts and Public Health England, and we’ve been monitoring and we’ve been putting in place the targeted action that has worked in other cases.
“In this case, it hasn’t worked over that period and so we needed to take further action which we took last night.”
Additional reporting by Press Association