The United States said it will send an aircraft to Japan to bring back US passengers on the quarantined cruise ship Diamond Princess, where the most coronavirus infections outside China have occurred.

The US Embassy in Tokyo said in a letter on Saturday to passengers that a chartered plane would arrive in Japan on Sunday evening and that it recommended "out of an abundance of caution" that US citizens disembark and return home for further monitoring.

The passengers would be required to undergo further quarantine of 14 days upon arriving in the United States and if they choose not to return on the flight, they would not be able to return home "for a period of time", the letter said.

"We understand this is frustrating and an adjustment, but these measures are consistent with the careful policies we have instituted to limit the potential spread of the disease," it said.

It also said passengers would be screened before the flight and the US government was working with Japan so that any people with symptoms would receive proper care if they could not board the plane.

The cruise ship, owned by Carnival Corp (CCL.N), has been quarantined since arriving in Yokohama on February 3, after a man who disembarked in Hong Kong before it traveled to Japan was diagnosed with the virus.

It had some 3,700 passengers and crew on board. Another 67 people have tested positive for the virus, Japanese Health Minister Katsunobu Kato said on Saturday, bringing the total to 285 cases. Those testing positive are transferred to Japanese hospitals.

Japanese public broadcaster NHK has reported that there were more than 400 US citizens on board.

The cruise liner's quarantine is set to end on Wednesday and while some passengers were disheartened at the prospect of more time in quarantine, others were more understanding.

"They are very concerned about spreading the virus, and there's no good way to transport people from Japan without possible transfer of virus, so it is the logical thing to do," Sawyer Smith, 25, told Reuters.

The plane will land at Travis Air Force Base in California and some passengers will then continue onward to Lackland Air Force Base in Texas.

The letter did not specify how long US citizens who choose not to board the chartered flight might have to wait before they could return home, saying only that the final decision would be up to the Centers for Disease Control and Prevention.