WASHINGTON (AFP) - Americans should avoid travelling by cruise liner within Asia because the vessels act as amplifiers of the novel coronavirus, a senior US official said on Friday (Feb 21), adding that future evacuations of ship passengers were not guaranteed.

The warning came as the number of confirmed cases of people on US soil who have been infected with the pathogen rose to 34, 21 of them repatriated from abroad.

More than 300 Americans were flown back from the Diamond Princess cruise ship off the Japanese coast earlier this week, and there have also been several flights bringing home citizens from the Chinese city of Wuhan, where the virus first emerged in December.

Ian Brownlee, a senior State Department official, said in a briefing that the government had "advised all US citizens to reconsider travel by cruise ship to, or within Asia."

"While the US government has successfully evacuated hundreds of hundreds of our citizens in recent weeks, such repatriation flights do not reflect our standard practice and should not be relied upon as an option for US citizens under potential risk of quarantine by local authorities," he added.

Eighteen of the passengers from the Diamond Princess ship have so far tested positive for the virus and were being treated, either at a medical centre in Nebraska or at hospitals near air bases in California and Texas.

Three people repatriated from Wuhan have also tested positive.

Nancy Messonnier, a senior official with the Centres for Disease Control and Prevention, said she expected the number of cases to rise among those who had returned from the Diamond Princess, because of their close proximity to one another while on board the ship.

There are also many Americans with the Covid-19 illness who are hospitalised in Japan, several of whom are seriously ill, she added.

Related Story Interactive: What we know so far about the coronavirus

"We're not seeing community spread here in the United States yet, but it's very possible, even likely that it may eventually happen," she warned.

The World Health Organisation said Friday that the window to stem the epidemic was shrinking, amid concern over a surge in cases with no clear link to China.

The outbreak has already killed more than 2,200 people and infected more than 75,500 in China.