New clusters emerge in countries praised for successful coronavirus fights.

“Several nations that have been hailed for curbing the spread of the pandemic are finding that victory over the coronavirus can be elusive and fleeting — especially after they start lifting lockdowns.

Singapore was once a model for its speed and efficiency in tracing the contacts of infected people. But cases there have ballooned to more than 23,000 as the virus spread in dormitories for foreign workers.

Officials in Wuhan, the Chinese city where the outbreak began, recently celebrated its emergence from more than two months in lockdown. But after six new cases were confirmed this week, it said it would test all 11 million of its residents.

And in South Korea, which has been a pioneer in using technology for contact tracing, more than 100 new cases have emerged after an infected man visited bars and clubs in a Seoul nightlife district. Just days after some restrictions were relaxed, the mayor ordered nightspots closed indefinitely.

Officials from the World Health Organization are urging governments and the public to maintain “extreme vigilance” while easing restrictions and that ignoring precautions would inevitably lead to more infections. Tedros Adhanom Ghebreyesus, the director-general of the W.H.O., said on Monday that only a “slow, steady lifting of lockdowns” could ease economic pains while keeping infections at bay.

In France, drinking alcohol was banned on Paris canals and riverbanks on Monday, after throngs of people were seen drinking along the Canal Saint-Martin as the country eased strict confinement rules. Germany, despite an aggressive program of testing and tracing infections, has seen an increase in cases since measures were eased last week.

In the United States, scientists are warning that the feared “second wave” could instead consist of a series of smaller waves breaking across the country.

Dr. Tedros said that social distancing restrictions were still the best weapons against the virus. But the W.H.O. also said on Tuesday that some treatments, in early studies, appear to be limiting the severity or length of Covid-19.

“We do have potentially positive data coming out but we need to see more data to be 100 percent confident that we can say this treatment over that one,” Margaret Harris, a spokeswoman for the W.H.O., said at a briefing, according to Reuters news agency.”