Thousands of stranded tourists were free to disembark from a cruise ship at an Italian port after authorities said two Chinese passengers feared to have the coronavirus tested negative Thursday.

Italy's institute for infectious diseases examined samples taken from a Chinese woman ill with a fever, as well as her travelling companion, and both were negative for coronavirus, the health ministry said.

Italian Prime Minister Giuseppe Conte however stopped all flights between Italy and China following the discovery that two Chinese tourists in a separate case had been confirmed as having the deadly SARS-like virus.

As a result of the outbreak Italy has added screening measures at its airports
As a result of the outbreak Italy has added screening measures at its airports AEROPORTO DI ROMA / Handout

Over 6,000 tourists had spent around 12 hours in lockdown on the Costa Smeralda. Some of the 1,400 or so passengers due to permanently disembark in Civitavecchia began getting off late Thursday.

Some 1,000 new passengers were due to board in the port near Rome, and Costa Cruises said the liner would set sail again on Friday.

The news came as the UN health agency declared an international emergency over the coronavirus from China, which reported its biggest single-day jump in deaths.

Costa Cruises said a 54-year old woman from Macau "was placed in solitary confinement in the on-board hospital last night with her travel companion".

The Costa Crociere liner carrying some 7,000 people is in lockdown at the Italian port of Civitavecchia
The Costa Crociere liner carrying some 7,000 people is in lockdown at the Italian port of Civitavecchia AFP / Filippo MONTEFORTE

Some 750 other Chinese passengers were on board and all appear to be in good health.

"People are irritable and exasperated," one passenger who was on board with his wife told ANSA news agency, before the go-ahead to disembark. Another described it as "a very stressful day".

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

A 54-year old woman from Macau has been put in solitary confinement in the on-board hospital with her travel companion
A 54-year old woman from Macau has been put in solitary confinement in the on-board hospital with her travel companion AFP / Filippo MONTEFORTE

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

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 skip that stop and head straight for Savona, the media said.

An initial plan to disembark those who had been due to leave the ship Thursday morning had been 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 percent Thursday morning.

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.

Coronavirus China