South Korea, China and Germany have reported fresh surges in coronavirus cases after easing their lockdown measures.
Authorities have warned that the scourge could come back with a vengeance without widespread testing and tracing of infected people's contacts with others.
"The nation is at risk," Park Won-soon, the mayor of the South Korean capital, Seoul, said on Monday, warning that the next few days would be "critical" in preventing the spread of a virus from a cluster of cases linked to several of the city's nightclubs and bars.
A total of 86 new infections have been reported in the new outbreak as officials race to track down thousands of others who may have come into contact with a 29-year-old man who visited the venues before testing positive for Covid-19.
Fears of infection spikes in countries that have loosened up have been also borne out in recent days in Germany, where new clusters were linked to three slaughterhouses; and in Wuhan, the Chinese city where the crisis started.
The scares will ring alarm bells for other governments eager to loosen lockdown restrictions.
South Korea has won global praise for successfully controlling the virus with its efficient "test, track, treat" strategy, reducing new infections to a daily trickle of single digit figures. The sudden spike in cases has raised fears of a second coronavirus wave.
The authorities have tested more than 2450 people who went to the night spots in the Itaewon neighbourhood, but officials are still trying to track about 3000 more with the help of phone records and credit card data.
Entertainment venues require customers to provide their contact details at the entrance for tracing purposes in the event of an outbreak.
However, Park said he believed some who were not accounted for may have given false details.
The contact tracing operation has been complicated by the fact that many of the new cases have been linked to venues catering to members of the LGBT community.
Homosexuality is often treated as a taboo subject in South Korea's conservative society, where LGBT people face discrimination.
The new cluster has highlighted difficulties with the country's invasive tracing methods and its disclosure of some patient information, including their recent locations, and the authorities have sought to reassure those who come forward that their privacy will be protected.
The latest outbreak has been a cruel blow for Asia's fourth largest economy, which was just starting to loosen work-from-home precautions and is set to reopen schools in phases from this week. This is still set to go ahead, although the government has cautioned it could be reversed if the disease spreads.
On Sunday, Moon Jae-in, the South Korean president, asked citizens to neither panic nor let down their guard, but he warned that the country was in a "prolonged war" against the virus.
"It's not over until it's over," he said in a televised address.
A cluster of new cases reported on in the Chinese city of Wuhan, where the pandemic began, is another worrying twist.
After a severe 76-day lockdown, Wuhan, a city of 11 million, had only begun to return to relative normality late last month, but on Monday it announced a new cluster of five cases, all people who had been infected locally.
The ability of the disease to spread undetected in such a heavily surveilled city raises concerns about the prospects for quickly returning to normality.
"At present, the task of epidemic prevention and control in the city is still very heavy," said the Wuhan health authority in a statement. "We must resolutely contain the risk of a rebound."
Over the weekend, China was also forced to impose a new lockdown on Shulan, a city of 700,000 bordering North Korea in the northeastern province of Jilin.
The new restrictions have raised suspicions about a coronavirus outbreak in the isolated hermit kingdom, which has so far denied any infections. Its claim to have zero cases was met with scepticism, and concerns that the impoverished nation would not be able to cope with a national epidemic.
Xi Jinping, the Chinese President, at the weekend offered to provide support to Kim Jong-un to fight the virus. China has already sent an unspecified number of Covid-19 testing kits to its neighbour.