China and South Korea have reported new spikes in coronavirus cases, setting off fresh concerns in countries where outbreaks had been in dramatic decline.
Meanwhile, new protests against pandemic restrictions erupted in Germany despite the easing of many lockdowns in Europe.
In the United States, former president Barack Obama harshly criticised his successor Donald Trump's handling of the coronavirus pandemic as an "absolute chaotic disaster".
The US has recorded 1.3 million infections and nearly 80,000 deaths in the pandemic, the most in the world by far, according to a tally by Johns Hopkins University in Maryland.
Worldwide, health officials are anxiously watching to see just how much infection rates rise in a second wave as nations and states emerge from varying degrees of lockdown.
China reported 14 new cases yesterday, its first double-digit rise in 10 days.
Eleven of 12 domestic infections were in the north-eastern province of Jilin, which prompted authorities to raise the threat level in one of its counties, Shulan, to high risk, just days after downgrading all regions to low risk.
Authorities said the Shulan outbreak originated with a 45-year-old woman who had no recent travel or exposure history but spread it to her husband, her three sisters and other family members.
Train services in the county were suspended.
"Epidemic control and prevention is a serious and complicated matter, and local authorities should never be overly optimistic, war-weary or off-guard," said the Jilin Communist Party secretary, Bayin Chaolu.
Jilin also shares a border with North Korea, which insists it has no virus cases, much to the disbelief of international health authorities.
South Korea reported 34 more cases as new infections linked to nightclub-goers threatens the country's hard-won gains against the virus.
It was the first time that South Korea's daily infections were above 30 in about a month.
President Moon Jae-in said citizens must neither panic nor let down their guard, but warned that "the damage to our economy is indeed colossal as well".
Across Europe, many nations are easing lockdowns even further today even as they prepared to clamp down on any new infections.
Germany, which managed to push daily new infections below 1,000 before deciding to loosen restrictions, has seen regional spikes in cases linked to slaughterhouses and nursing homes.
On Saturday, outbreaks at several meat-packing plants in North Rhine-Westphalia - the country's most heavily populated state - prompted the local leadership to promise to test each of the estimated 18,000-20,000 meat workers in the state. In the western town of Coesfeld, where 151 of 200 slaughterhouse workers tested positive for the virus, authorities decided to suspend lockdown relaxations.
Despite the local outbreaks, Armin Laschet, the prime minister of North Rhine-Westphalia, called for the country's border with France to be reopened in order to foster European solidarity.
Chancellor Angela Merkel struck a different tone when announcing the first phase of relaxations in midweek, reminding the German public "we still have a long fight against the virus ahead of us".
German officials have also expressed concerns about the growing number of large demonstrations, including one in the south-western city of Stuttgart that drew thousands of participants.
Police in Berlin stepped in on Saturday after hundreds of people failed to respect social-distancing measures at anti-lockdown rallies in the German capital.
Ms Merkel and the governors of Germany's 16 states last week cleared the way for restaurants, hotels and remaining stores to reopen.
The country's football league resumes next week, despite a number of professional players testing positive for Covid-19, and more students are returning to school beginning today.
In Spain, residents in some regions will be able to enjoy limited seating at bars, restaurants and other public places beginning today but Madrid and Barcelona, the country's largest cities, will remain shut down.
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