Live updates: follow the latest news on Covid-19 variant Omicron
The US Centres for Disease Control and Prevention has advised Americans against travelling to France and Portugal as concerns over the Omicron coronavirus variant grow.
The CDC now lists 83 destinations at "Level 4: Very High" classification and also added Jordan, Tanzania, Andorra, Cyprus and Liechtenstein to the highest travel advisory level.
Countries that fall in the highest category have an incidence rate of more than 500 new coronavirus cases over the past 28 days for populations of over 100,000, or 500 cumulative new cases over 28 days for countries with under 100,000 people.
"Because of the current situation in France, even fully vaccinated travellers may be at risk for getting and spreading Covid-19 variants," reads one CDC travel advisory.
France said on Monday it would close nightclubs ahead of Christmas and tighten social distancing measures in response to the emergent Omicron variant, but that there was no need for new lockdowns or curfews.
Prime Minister Jean Castex said a fifth wave of the pandemic was surging, but with 52 million people now vaccinated - nearly 90 per cent of those eligible - the situation was better than in previous outbreaks.
France has identified 25 Omicron cases to date, according to its health minister.
The US instituted new rules for travel, requiring all international air travellers show proof of a negative Covid-19 test within on day of departure, and US President Joe Biden extended mask mandates on air travel for international and domestic flights through the winter months.
All inbound travellers must also be fully vaccinated.
Under the prior rules, vaccinated international air travellers could present a negative test result obtained within three days of their day of departure.
Last month the US barred inbound travel from eight southern African nations after the Omicron variant was first detected in South Africa. The US has not imposed further travel bans to other countries where the new variant has been reported.
Reuters contributed to this report