He, however, urged countries to "do more in spite of this clear progress", adding that it was coordinating a "vast range of research endeavours" to investigate different treatment options for Covid-19.
"Let's prevent this tragedy", he said.
The WHO Director-General Tedros Adhanom Ghebreyesus didn't specify why he believes the outbreak that has infected some 2.5 million people and killed over 166,000 could get worse. "It's a virus that many people still don't understand", he added.
He announced a decision to halt funding for the United Nations agency earlier this month; congress controls the funds.
Dr Tedros and his agency have been on the defensive after US President Donald Trump last week ordered a halt to his country's funding of the agency, alleging it botched the early response to the outbreak.
Bayfield County Health Department confirms first COVID-19 death
That number is more than the combined total of the next four countries with the most cases (Spain, Italy, Germany, France). With no significant travel history, the man developed a fever, dry cough, fatigue and loss of taste, according to Dr.
Ghebreyesus didn't explain exactly how he expects the virus' toll to intensify, but he argued that partisanship and political divisions are exacerbating the crisis. "Had the World Health Organization done its job to get medical experts into China to objectively assess the situation on the ground and to call out China's lack of transparency, the outbreak could have been contained at its source with very little death".
Some Asian and European governments have gradually eased or started relaxing lockdown measures such as quarantines, school and business closures and restrictions on public gatherings, citing a decline in the growth of COVID-19 case counts and deaths. Nevertheless, Tredros said: "There is no secret in World Health Organization because keeping things confidential or secret is risky".
She also said the complaint did not detail the alleged harm suffered by the individual plaintiffs, or show what legal duty the World Health Organization owed to them.
"This virus is risky".
"Most epidemics in Europe appear to be stable or declining, although numbers are low we see worrying upward trends in Africa, Central and South America and Central Europe", he said. "It exploits cracks between us when we have differences".