International passengers heading to the US will have to be tested for COVID-19 within one day of travel regardless of their vaccination status, as the Omicron variant spreads around the world.
The tightening of coronavirus rules by the White House comes after America reported its first known case of community transmission of the highly contagious new strain.
Other COVID-19 measures outlined by US President Joe Biden include extending the requirement to wear masks on planes, trains and buses to March.
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With countries around the world scrambling to contain the Omicron variant, Mr Biden warned that infections will rise this winter.
“We’re going to fight this variant with science and speed, not chaos and confusion,” he said.
“The actions I’m announcing are ones that all Americans can rally behind and should unite us in the fight against COVID-19.”
New York has found five cases of the new variant, its governor said, bringing the total number of infections in the country to eight.
The other US states that have identified cases are California, Colorado and Minnesota, one in each state.
The case in Minnesota is the first known community transmission within the US, with the patient having recently travelled to New York City for a convention, prompting the launch of contact tracing to try to contain the spread.
Much remains unknown about Omicron, which was first detected in southern Africa last month and has been reported in dozens of countries, just as parts of Europe were already struggling with a wave of infections of the dominant Delta variant.
However, the first real-world study of the variant’s effect has found it has a “substantial” ability to evade immunity from a previous COVID infection.
The US administration is also requiring private health insurers to refund customers for the total cost of over-the-counter home testing COVID-19 kits.
Tests will also be made available for free at rural clinics and health centres for the uninsured.
Less than 60% of the US population have been fully vaccinated, one of the lowest rates among wealthy nations.
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Fears about the Omicron variant have also hit financial markets and cast doubt on the speed of the global economic recovery, by fuelling supply chain problems and dampening demand.
US Treasury Secretary Janet Yellen said: “There’s a lot of uncertainty, but it could cause significant problems. We’re still evaluating that.”