The new rules, which comes into effect Monday, means travellers from the US will be subject to the highest tier of quarantine measures in the semi-autonomous city, which has some of the strictest anti-Covid policies in the world.

They will make the US the first country outside of Africa to be placed under a quarantine centre order since the Omicron variant was first detected last month.

Travellers from the US will only be allowed to enter the city if they are fully vaccinated Hong Kong residents and will be required to spend their first seven days at a quarantine centre with daily testing and health monitoring.

Afterwards, they will need to spend another 14 days in quarantine at a hotel booked in advance.

US travellers entering Hong Kong are already required to quarantine for a total of 21 days, but under current rules, they can remain in a hotel for the duration of their isolation.

The Hong Kong government said Friday it would tighten its rules after confirming a new Omicron case -- the city's fifth -- involving a 37-year-old man arriving from the United States.

"The whole genome sequencing conducted by the Department of Health's Public Health Laboratory Services Branch confirmed the case to be an Omicron case," it said.

Hong Kong has followed China's lead in adopting a "zero-Covid" strategy, which has led to some of the toughest quarantine and testing measures in the world.

The policy has kept infections low but ensured a business hub that dubs itself "Asia's World City" has been cut off internationally for the past 21 months.