In a statement explaining the evidence, the US Centers for Disease Control and Prevention cited the rise in infections and what it said was a lack of adequate and transparent information from China, including genomic sequencing of viral strains circulating in the country.

"These data are critical to effectively manage the increase in cases and reduce the potential for the introduction of a new variant of concern," the CDC said.

Some scientists are concerned that the increase in COVID-19 cases in China could trigger a new variant of coronavirus in the world, which may or may not be similar to those currently circulating. This is because each infection is a new opportunity for the virus to mutate.

Under the new US rules, travellers from China, Hong Kong and Macau to the US must be tested for COVID-19 no later than two days before travel and test negative before boarding the flight. The test applies to anyone 2 years of age or older, including US citizens.

It will apply to persons travelling from China via a third country and to persons making a stopover in the United States on their way to other destinations. Any person testing positive more than 10 days before the flight will be able to present documentation showing that they have recovered from COVID-19 rather than a negative result.

It will be the responsibility of the airlines to confirm negative results and documentation of recovery prior to passengers boarding.

Chief Executive John Lee announced on Tuesday that Hong Kong will finally fully open its doors from Wednesday, ending some of the world's toughest COVID-19 measures.

Lee said Hong Kong will cancel mandatory PCR testing for incoming travellers, the Vaccine Pass scheme, quarantine requirements for close contacts and social distancing measures.

The government had already cancelled quarantine requirements for arrivals at the end of September. The only remaining measure is the requirement to wear masks.

This measure is expected to bring back much-needed Chinese tourism from next month, during the Lunar New Year holidays.

Before the pandemic, tourism accounted for 4.5 per cent of Hong Kong's GDP and employed about 257,000 people, accounting for about 6.6 per cent of total employment, according to government figures. In 2018, of the 65 million tourists who visited Hong Kong, 78% - 51 million - were mainlanders.

Restrictions were lifted, but an "emergency brake" was maintained that could be activated if necessary to reintroduce restrictions in a coordinated manner.

These updates were agreed by member states on 7 December, the spokesperson said, explaining that it was agreed to revert to a pre-pandemic approach of free movement and travel without any covid-19-related measures.

"Should the epidemiological situation require it, covid-19 measures could be reintroduced in a coordinated manner and following a people-based approach," he said.

He also noted that the omicron BF7 variant of the coronavirus, prevalent in China, is already present in Europe and "has not grown significantly".

"However, we remain vigilant and will be ready to use the emergency brake if necessary," he concluded.

A passenger holds a passport issued by the Chinese government.

US, several countries announce new measures to control arrivals of travellers from China

The United States on Wednesday joined Italy, India, Taiwan, South Korea, Malaysia and Japan in announcing new measures after China recorded an exponential rise in cases of covid after relaxing its strong measures against the coronavirus.

In justifying the move, the US Centers for Disease Control and Prevention cited rising infections and a lack of adequate and transparent information from China, including genomic sequencing of strains.