Passengers wear masks as they prepare to travel from Rome's international airport in Fiumicino. AP

The United States told its citizens not to travel to China in the first major response to the World Health Organization's declaration of a global emergency over the spreading coronavirus, as Chinese authorities Friday increased the toll to 213 dead and nearly 10,000 infections.

The State Department raised it warning alert to the highest level, telling its citizens "do not travel" to China because of an epidemic that has now spread to more than 20 nations.

Hours earlier, the WHO, which has faced criticism for initially downplaying the virus threat, revised its risk assessment after crisis talks in Geneva.

"Our greatest concern is the potential for the virus to spread to countries with weaker health systems," said WHO chief Tedros Adhanom Ghebreyesus.

British health officials said Friday two people had tested positive for the novel coronavirus, in the country's first cases since the deadly outbreak emerged in China and spread globally this month.

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The announcement came as Britain prepares to receive 87 of its citizens it has evacuated from Wuhan, the Chinese city at the centre of the epidemic which has so far killed 213 people.

The Italian government declared a state of emergency on Friday to fast-track efforts to prevent the spread of the deadly coronavirus strain after two cases were confirmed in Rome.

Italy had said Thursday it was stopping all flights to and from China following the news that two Chinese tourists holidaying in Italy had tested positive for the virus.

Meanwhile, more than 6,000 tourists were under lockdown aboard a cruise ship at an Italian port on Thursday after two Chinese passengers were isolated over fears they could be carrying the coronavirus.

Preliminary tests on the pair suggested they were not carrying the virus, the ministry of health said, but it was not immediately clear whether passengers would be allowed to leave the Costa Smeralda vessel before final results were confirmed.

The Lazzaro Spallanzani national institute for infectious diseases is examining samples taken from a Chinese woman who fell ill with a fever as well as her travelling companion. It told AFP that the definitive results could take up to 48 hours.

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It came as China reported its biggest single-day jump in coronavirus deaths and global fears deepened over a spread of the disease, with at least 15 countries confirming infections.

The Costa Smeralda cruise ship is docked in the Civitavecchia port 70km north of Rome. AP

Cruise line Costa Crociere confirmed the ship, which is currently in the port of Civitavecchia, was on lockdown with some 7,000 people in total including the crew on board.

Costa said a 54-year-old woman from Macau "was placed in solitary confinement in the on-board hospital last night with her travel companion," and the crew was following instructions from the health ministry.

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Some 750 other Chinese passengers are on board and all appear to be in good health, media reports said.

The Smeralda, the company's flagship and the fifth-largest cruiser in the world, had sailed in from Palma de Mallorca as part of a one-week trip in the western Mediterranean.

Italy's Prime Minister Giuseppe Conte earlier said the government was "not worried, but vigilant and cautious."

Adriano Pavan, whose son and two grandchildren were on board and who had travelled to the port to meet them off the ship, told AFPTV that he was "quite worried."

"My son told me the Chinese couple had been wearing masks for the past two or three days, but they thought nothing of it".

In isolation

The couple flew in to Milan from Hong Kong on Jan.25, before getting on board at Savona in northern Italy, according to media reports.

"The couple's cabin has been isolated and they are in with the doctors," an unnamed passenger was quoted as telling ANSA news agency.

"We're a bit worried of course. No-one is getting on or off the ship apart from the doctors. This holiday risks ending in a nightmare."

The ship also stopped at Marseilles and Barcelona, as well as Parma de Mallorca, according to an itinerary on its website.

It had been due to sail for La Spezia in north Italy late Thursday, but would be detained in the Civitavecchia port near Rome "until we have an update on the health tests," said port captain Vincenzo Leone.

An initial plan to disembark some 1,140 passengers who had been due to leave the ship on Thursday morning was put on hold after Civitavecchia mayor Ernesto Tedesco protested.

"Are you mad? Who gave you the order to disembark? I'll take you all to court," he shouted at officials after racing to the port in his Fiat 500, according to the Repubblica daily.

Carnival, the company which owns Costa Crociere, saw its stock price plunge nearly 11 per cent on Thursday morning.

The World Health Organisation, which initially downplayed a disease that has now killed 170 in China, was preparing to meet on Thursday to decide whether to declare it a global emergency.

Silvio Brusaferro, president of the Italian national health institute, said that all suspected cases in Italy so far had turned out to be false alarms, but the health service was "ready should any cases of infection emerge".

Beijing has taken extraordinary steps to halt the spread of the virus, including effectively quarantining more than 50 million people in Wuhan and surrounding Hubei province.

The pathogen is believed to have emerged in a market that sold wild game, and spread by a Lunar New Year holiday season in which hundreds of millions of Chinese travel domestically or abroad.

Agence France-Presse