Americans will be removed from a cruise ship quarantined off Japan and flown home, the US embassy said on Saturday as dozens of more cases of the new coronavirus were diagnosed on board.

At least 285 people on the Diamond Princess have contracted the illness but hundreds of passengers and crew have not yet been tested as they wait in a quarantine that was scheduled to end February 19.

In a message to Americans, the embassy said the US government “recommends, out of an abundance of caution, that US citizens disembark and return to the United States for further monitoring”.

The remarks were later echoed by a US state department spokesperson, who confirmed that US citizens should return.

The embassy warned that those who come back will have to undergo another 14 days of quarantine when they arrive in the United States.

A chartered flight will arrive in Japan on Sunday, but it was not immediately clear when it would leave. The message said the flight will land at Travis Air Force Base in California, with some passengers continuing to a second air base in Texas.

Japanese public broadcaster NHK said 400 Americans were on board the cruise ship and the plane would 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 warned.

The Japanese government was arranging special embarkation procedures for US citizens taking the chartered flight, an official said, adding that “a certain number” of other countries have also shown interest in similar evacuation measures.

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

“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?”