Countries are considering putting away their welcome mats to tourists as the coronavirus pandemic strengthens its grip in many of the worst-affected countries and creeps back into pockets in others that had controlled the virus.
Some countries that were preparing to let limited international travel resume are already reconsidering, as clusters of cases grow into new outbreaks.
Some European nations were warning citizens not to visit Spain after some of its most beloved summer venues turned into coronavirus hot spots facing renewed pandemic lockdowns.
The northeast regions of Catalonia and Aragón host the three most alarming virus clusters in Spain, prompting authorities to tighten restrictions in Barcelona, in a rural area around Lleida and in Zaragoza that were relaxed only a month ago when Spain had its devastating outbreak in check.
The British government suddenly re-imposed over the weekend a mandatory two-week quarantine for anyone entering the U.K. from Spain. The move angered airlines and holidaymakers alike, coming just a few weeks after restrictions were lifted, prompting a huge flood of summer travel bookings which have now been left in jeopardy.
The U.K. was reportedly closely monitoring coronavirus caseloads in Germany and France, too, and a government official said Monday that quarantine rules for arrivals from those two nations couldn't be ruled out in the coming days or weeks.
In Asia, existing outbreaks have been linked to cross-border travel.
South Korea said 16 of the 25 new cases it confirmed Monday were tied to people arriving from abroad.
The country in past days has reported dozens of cases among crew members of a Russia-flagged cargo ship docked in the southern port of Busan and hundreds of South Korean construction workers airlifted from virus-ravaged Iraq.
One of South Korea's recent new cases was a traveler who arrived last week from New Zealand, which has not had a community-transmitted case in three months. Any proof the passenger caught the virus while in New Zealand would come as a big shock to the island nation of 5 million people.
New Zealand health authorities said they will trace and test people who came into contact with the traveler. They are also asking the traveler be retested.
New Zealand officials said their South Korean counterparts suspect the traveler was infected while transiting through Singapore. "We have got our contact tracing system kicking into gear though," Health Minister Chris Hipkins said.
The number of worst-affected countries where newly confirmed cases are rising still outnumbers countries finally seeing a downward trend in infections, according to data compiled by Johns Hopkins University.
Its tally shows more than 16.2 million cases of COVID-19 and more than 648,000 deaths. The actual numbers are thought to be much higher due to limits to testing and other issues.