More than 40 Americans on a quarantined cruise ship in Japan have been infected with the coronavirus, a senior US health official has said.

The group began leaving a quarantined cruise ship a few hours ago to board chartered flights home as the number of new coronavirus cases diagnosed on the vessel jumped to 355.

The evacuation came as Japanese authorities stepped up warnings over the deadly outbreak, urging citizens to avoid crowds and "non-essential gatherings".

Earlier, the US Centers for Disease Control and Prevention (CDC) said that 400 Americans on the Diamond Princess cruise ship would be evacuated and flown back to the United States where they would face a 14-day quarantine period. 

But Anthony Fauci, a senior official at the National Institutes of Health, said that those confirmed with infections would not be allowed to fly. 

"They are not going to go anywhere," he said. "They're going to be in hospitals in Japan."

Asked about the severity of the infected Americans' illness, Mr Fauci said it varied.

"You could be infected and have minimal symptoms, but you still have the possibility of transmitting it to someone else," he said. 

He added that anyone showing potential symptoms of COVID-19, the disease caused by the novel coronavirus, would also not be able to get on evacuation planes.

"If people on the plane start to develop symptoms, they'll be segregated within the plane. So there's a very firm plan with this 747 that is going to take these passengers now who have been there."

Some American passengers from the cruise ship have indicated they would decline to be evacuated because they were asymptomatic. 

But Mr Fauci said everyone who had been on the ship would still be subject to quarantine on their return to the US.

"And the reason for that is that the degree of transmissibility on that cruise ship is essentially akin to being in a hot spot. A lot of transmissibility on that cruise ship," he explained.

The Diamond Princess was placed in a 14-day quarantine in early February after a former passenger tested positive for the virus.

But US authorities announced yesterday they would offer Americans on board the option to leave the ship and fly home, where they will face another 14-day isolation period. Several other governments have announced plans to remove their citizens from the ship as well.

Those who opted to leave were brought off the ship in groups, passing through a makeshift passport control but undergoing no health checks, US passenger Sarah Arana said.

However, other Americans on board declined the evacuation, despite being warned they will still have to wait two weeks and test negative for the virus before being allowed back to the United States.

"My health is fine. And my two-week quarantine is almost over. Why would I want to be put on a bus and a plane with other people they think may be infected when I have spent nearly two weeks isolated from those people?" tweeted Matt Smith, a lawyer on the ship with his wife.

Planes await US passengers who opted to leave the Diamond Princess ship

He described a fellow American passenger standing on her balcony chanting "USA, USA" as buses arrived to collect them.

"Of course, in contravention of the rules of quarantine, she's not wearing a face mask, and she's talking with a passenger on the adjacent balcony... And you wanted me to get on a bus with her?"

Earlier, Japan's health minister Katsunobu Kato said 1,219 people on the ship had now been tested for the virus, with 355 diagnosed with the illness.

Japan has not been able to test all those on board due to limited supplies of testing kits, facilities and manpower, which are also needed by authorities tracking the spread of the virus on land.

But the health ministry said that passengers older than 70 are being examined and those testing negative and in good health will be allowed to leave the ship from Wednesday.

Number of new cases drops third consecutive day

Meanwhile, the number of new cases from China's coronavirus epidemic dropped for a third consecutive day, as the World Health Organization chief warned it was "impossible" to predict how the outbreak would develop.

Global concern remains high about the spread of the virus, which first emerged in China's central Hubei province in December, with the first death outside Asia reported in France this weekend.

The death toll jumped to 1,665 in mainland China after 142 more people died from the virus.

More than 68,000 people have now been infected, but the number of new cases of the COVID-19 strain continued to decline.

In hardest-hit Hubei, the number of new cases slowed for a third consecutive day and at 139, the number of deaths was level with Saturday's toll.

The number of new cases in other parts of the country has dropped for 12 straight days.

Mi Feng, National Health Commission spokesman, said that the figures were a sign that China was controlling the outbreak.

"The effects of epidemic prevention and control in various parts of the country can already be seen," he told reporters.

But WHO chief Tedros Adhanom Ghebreyesus has warned that it was "impossible to predict which direction this epidemic will take".

"We ask all governments, companies, and news organisations to work with us to sound the appropriate level of alarm without fanning the flames of hysteria," he said, speaking at the Munich Security Conference.

The UN health body has asked China for more details on how diagnoses are being made.

An international team of WHO experts will arrive in Beijing this weekend for a joint mission with Chinese counterparts.

The scale of the epidemic ballooned on Thursday after authorities in Hubei changed their criteria for counting cases, retroactively adding 14,000 cases in a single day.

Chinese authorities have placed some 56 million people in Hubei and its capital Wuhan under quarantine, virtually sealing off the province from the rest of the country in an unprecedented effort to contain the virus.

Local authorities around China have introduced measures to try to stop the virus spreading.

Beijing's municipal government has enacted a rule requiring people coming to the capital to self-quarantine for 14 days, according to official media.

Outside mainland China, an 80-year-old Chinese tourist in France was the fourth person to die from the new coronavirus, with the other deaths in the Philippines, Hong Kong, and Japan.

Several countries have banned arrivals from China and major airlines have cut services to the country.

Malaysia said it would not allow any cruise ships departing or transiting Chinese ports to enter the country, following the discovery of a US citizen with the coronavirus.

The virus spread last month as millions travelled across China for the Lunar New Year holiday, which was extended to try to prevent more infections.

People have slowly started to return to work in the past two weeks, though many are doing their jobs from home and schools remain closed.

With the government facing criticism over its handling of the crisis, Chinese President Xi Jinping called for tighter policing to protect social stability.

The government must "increase use of police force and strengthen the visible use of police" during the crisis, Mr Xi said in a February 3 speech published by state media on Saturday.

A number of local officials have been sacked for their role in mishandling the outbreak, including Hubei's top two health officials, and the political chiefs of the province and Wuhan who were replaced with Xi loyalists.