Countries that claimed to have had coronavirus outbreaks under control have started seeing large numbers of new cases after easing their lockdowns, the World Health Organization has said.
Margaret Harris, a WHO spokesperson, urged people on Tuesday to keep themselves informed as COVID-19 cases surge again in some countries.
"We're seeing a lot of upticks, a lot of changes in different countries, countries that had successfully shut down their first transmission are seeing second upticks," she added, mentioning Australia and Hong Kong.
"If it's anywhere, it's everywhere and people traveling have to understand that," Harris continued.
"This virus is widespread and people have to take that very, very seriously."
In the meantime, travellers should "remember things will change, or may well change," Harris said at a Geneva briefing.
It comes as fresh coronavirus restrictions have been imposed in Australia following a surge in cases.
Lockdown measures were reimposed in Melbourne on Tuesday, confining residents to their homes unless undertaking essential business.
The UK and other countries in western Europe have loosened their lockdown restrictions over the last two months.
Pubs and restaurants in England were allowed to reopen on the weekend despite concern from some scientific advisers.
Travel to other countries is also being de-restricted, since the government announced last week a list of around 60 nations which British travellers could visit without the need to self-isolate on return.
The WHO said last month that it would update its own travel guidelines ahead of the northern hemisphere summer holidays but they have not yet been released.
In the meantime, travellers should "remember things will change, or may well change", Harris said at a Geneva briefing.
The WHO's previous guidance for travellers has included common-sense advice applicable to other settings such as social distancing, washing your hands and avoiding touching your eyes, nose or mouth.
Harris also proposed on Tuesday wearing a mask on planes, a measure which is already a requirement of many airlines.
"If you are flying, there is no way you can social distance in a plane, so you will need to take other precautions including using a face covering," she said.
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