“Let me be clear that we are not seeing community spread in the United States yet, but it is very possible, even likely, that it may eventually happen,” said Messonnier, director of the CDC’s National Centre for Immunisation and Respiratory Diseases.

“Our goal continues to be slowing the introduction of the virus into the US,” Messonnier said. “This buys us more time to prepare our communities for more cases and possibly sustained spread. This new virus represents a tremendous public health threat.”

Messonnier said the CDC believed Washington’s “aggressive travel precautions” – which include restrictions that virtually ban entry to mainland-based Chinese passport holders – were working, as the US was still not seeing evidence of the “community spread” that has hit several Asian countries.

Among the countries and territories that have detected cases without knowing the infection’s source are Singapore, South Korea, Taiwan, Thailand, Vietnam and Taiwan.

Americans on WHO team to assess coronavirus crisis, China says

The situation in South Korea, in particular, has become acute with authorities reporting 208 infections – quadruple the number two days ago. Two people have died of Covid-19, and Prime Minister Chung Se-kyun said the nation had entered “an emergency phase”.

Iran has reported 18 cases and four deaths in the last two days, while Lebanon on Friday reported its first infection – a woman who arrived from the holy city of Qom in Iran.

In Canada, health authorities in British Columbia said late on Thursday that they detected the virus in a woman who had no history of travel to China, where most of the nearly 78,000 infections and 2,250 deaths have occurred.

This incidence was a “sentinel event” that was a possible “indicator that there’s more widespread transmission”, provincial health officer Bonnie Henry said. Canada has recorded nine infections so far.

Separately on Friday, the World Health Organisation (WHO) said its team of public health experts – currently in China to help local authorities investigate the epidemic – was set to visit Wuhan, the outbreak’s epicentre.

WHO Secretary General Tedros Adhanom Ghebreyesus said in Geneva that the team was to travel to Wuhan on Saturday. It already has visited Sichuan, Beijing and Guangdong since arriving on the mainland last week.

Its 12 international specialists, including Americans, have been working with their Chinese counterparts to learn more about the virus, such as its transmission rate and the most effective treatment. Also on the team are experts from Germany, Japan, Nigeria, Russia, Singapore and South Korea.

US officials had previously expressed concern over China’s lack of response to their offers in early January to send experts to the mainland.

Tedros also urged the world’s governments to remain vigilant amid signs of community transmission in multiple countries.

“The cases that we see in the rest of the world, although the numbers are small, but not linked to Wuhan or China, it’s very worrisome,” Tedros said.

Asked if the epidemic had reached a tipping point, the WHO chief said he believed there remained a narrow “window of opportunity” to contain it – a phrase he has repeatedly used in recent weeks to beat back assertions that a global pandemic is inevitable.

“This outbreak could go in any direction,” he said. “If we do well, we can avert any serious crisis, but if we squander the opportunity, then we will have a serious problem on our hands.”