A surge of infections has prompted countries to reimpose some travel restrictions in recent days, with Britain throwing the reopening of Europe's tourism industry into disarray by ordering a quarantine on travellers returning from Spain.

Only with strict adherence to health measures, from wearing masks to avoiding crowds, would the world manage to beat the COVID-19 pandemic, the World Health Organization's director-general, Tedros Adhanom Ghebreyesus, said at a virtual news briefing.

"Where these measures are followed, cases go down. Where they are not, cases go up," he said, praising Canada, China, Germany and South Korea for controlling outbreaks.

WHO emergencies program head Mike Ryan said it was impossible for countries to keep borders shut for the foreseeable future.

"...It is going to be almost impossible for individual countries to keep their borders shut for the foreseeable future. Economies have to open up, people have to work, trade has to resume," he said.

"What is clear is pressure on the virus pushes the numbers down. Release that pressure and cases creep back up."

Ryan praised Japan and Australia for having had "good success in containing the disease" but said that it was to be expected that the virus would resurge in areas with active transmission if restrictions are lifted and mobility increased.

"And that is what has essentially occurred in many countries is that in nightclubs, other situations, dormitories, other environments in which people are close together can act as amplification points for the disease and then it can spread back into the community. We need to be hyper-alert on those."

Measures must be consistent and kept in place long enough to ensure their effectiveness and public acceptance, Ryan said, adding that governments investigating clusters should be praised not criticized.

"What we need to worry about is situations where the problems aren't being surfaced, where the problems are being glossed over, where everything looks good."

Ryan said Spain's current situation was nowhere near as bad as it had been at the pandemic's peak there, and he expected clusters to be brought under control, though it would take days or weeks to discern the disease's future pattern.

"The more we understand the disease, the more we have a microscope on the virus, the more precise we can be in surgically removing it from our communities," he added.