Americans on board a cruise ship quarantined off Japan since early February over coronavirus fears will be evacuated and returned to the US via chartered flight, officials said Saturday.

In a message to Americans on board the Diamond Princess, the US embassy said those returning to the United States would have to undergo another 14 days of quarantine.

"The US government recommends, out of an abundance of caution, that US citizens disembark and return to the United States for further monitoring," the statement said.

Washington will send a chartered plane to Japan for the evacuation on Sunday, the message said.

It was not clear when the plane will depart Japan, but the letter said it will land at Travis Air Force Base in California.

Japanese public broadcaster NHK said some 400 Americans were on board the cruise ship and the plane will leave Tokyo's Haneda airport as soon as early Monday.

"Passengers will be screened for symptoms and we are working with our Japanese partners to ensure that any symptomatic passengers receive the required care in Japan if they cannot board the flight," the message said.

"Should you choose not to return on this charter flight, you will be unable to return to the United States for a period of time," it added.

While some Americans on the ship have urged their government to extract them from the boat, others on board criticised the plan.

A US plane will carry Americans from the Diamond Princess cruise ship off Japan to Travis Air Force Base in California
A US plane will carry Americans from the Diamond Princess cruise ship off Japan to Travis Air Force Base in California AFP / CHARLY TRIBALLEAU

"Incredibly disappointed that the U.S. Government has decided to throw a monkey wrench into the quarantine we have maintained here on board the Diamond Princess," tweeted Matt Smith, an American lawyer.

"The U.S. Government... wants to take us off without testing, fly us back to the U.S. with a bunch of other untested people, and then stick us in 2 more weeks of quarantine? How does that make any sense at all?"

There were more than 3,700 people on the ship when it arrived off the Japanese coast in early February. It was placed into quarantine after a passenger who got off the boat in Hong Kong tested positive for the COVID-19 coronavirus.

Since it arrived off Japan, more than 200 others have since been diagnosed and taken off the ship to local hospitals.

The illness has killed more than 1,500 people and infected at least 66,000 in China while spreading to more than two dozen other countries, sparking an unprecedented containment effort.

On Friday, Japan's government gave passengers on the Diamond Princess aged 80 or older and in poor health the chance to move from the ship to quarantine accommodation on land.

The quarantine on the ship had been due to end on February 19 and those on board have been mostly confined to their cabins, asked to wear masks, and keep their distance from other passengers during brief outings on deck.

Crew on board have expressed concern that their conditions -- including shared cabins, bathrooms and workspaces -- put them at greater risk of contracting the virus.

Excluding the cases on the ship, and an infected quarantine officer, Japanese authorities have so far diagnosed at least 40 people with the virus.