The US authorities have revealed 14 of some 300 American citizens evacuated from a vessel of Princess Cruises are found to have infected with the Covid-19 coronavirus.

In a joint statement, the Department of State and the Department of Health and Human Services (HHS) said US officials received notice the 14 passengers were tested positive during the evacuation process.

“These individuals were moved in the most expeditious and safe manner to a specialised containment area on the evacuation aircraft to isolate them in accordance with standard protocols,” the statement said.

“After consultation with HHS officials…the State Department made the decision to allow the 14 individuals, who were in isolation, separated from other passengers, and continued to be asymptomatic, to remain on the aircraft to complete the evacuation process.”

“During the flights, these individuals will continue to be isolated from the other passengers.”

Two US government charter flights carrying Americans from the 115,900-gt Diamond Princess (built 2004) have departed Tokyo for the US.

Japanese authorities said on Monday that the number of new cases on board the ship had risen by 99 to 454, according to media reports.

On Sunday US citizens aboard the vessel had received a letter from US authorities advising them to disembark from the ship and return to the US.

Based on the high number of Covid-19 cases identified onboard the Diamond Princess, US authorities had said an assessment showed that passengers and crew members on board were “at high risk of exposure”.

Anthony Fauci, director of the US National Institute of Allergy and Infectious Diseases told US network CBS that the "degree of transmissibility on that cruise ship is essentially akin to being in a hot spot".

Evacuees left Yokohama Port around 1:40 am Monday aboard 10 coaches belonging to Japan's Self-Defense Forces, with drivers in protective suits, Kyodo news agency reported.

Passengers on the chartered aircraft will be quarantined in the US at Travis Air Force Base in Fairfield, California or Lackland Air Force Base near San Antonio, Texas for 14 days upon arrival.

These sites were specifically selected to ensure health care facilities are immediately available in the vicinity to treat any medical needs, US officials said.

At least 40 US citizens who were previously found to have infected with the virus will remain in Japan for treatment.

Some 400 Americans were reported to have been aboard the Princess Cruises vessel, which has been held in Yokohama since 3 February due to the coronavirus.

“We extend our thanks to the Government of Japan for its invaluable collaboration with us, and for its commitment to protecting the welfare of US citizens,” the US Embassy in Japan said.

“We also extend our sincere gratitude to the Self-Defense Forces for helping to transport our American citizens, a contribution in the true spirit of the Alliance.”

Canada, Hong Kong and Israel have also said they are preparing to send chartered flights to Japan to evacuate their citizens from the vessel.