TOKYO (AFP) - Two Japanese citizens evacuated from the epicentre of a novel coronavirus outbreak have been diagnosed with the infection after initially testing negative, the health ministry said on Tuesday (Feb 11).

One of the two, a man in his 50s who returned from the Chinese city of Wuhan on the first Japanese evacuation flight on Jan 29, previously twice tested negative for the virus.

However, a third test 12 days later found the man - who has been isolated in his hotel room since his return - was infected.

According to the World Health Organisation, the coronavirus incubation period "could be up to 14 days".

The second evacuee, a Japanese man in his 40s, returned from Wuhan on Jan 30 and initially tested negative for the virus, but was diagnosed after a second test on Monday.

The new cases bring the number of infections inside Japan to 28, excluding dozens diagnosed aboard a cruise ship moored off the coast.

So far, 135 people on board the Diamond Princess have tested positive for the virus since it arrived off Japan on Monday. Authorities on Tuesday were removing from the ship the most recently diagnosed 65 crew and passengers.

The ship was carrying 3,711 people when it arrived off Japan on the evening of Feb 3, and was officially placed into quarantine two days later.

Only those diagnosed with the virus or suffering other serious health conditions have been allowed off the boat, which is expected to remain in quarantine until at least Feb 19.

People on board the ship on Tuesday tweeted that medicines were being delivered to passengers, and that the ship's captain had announced the vessel would head out to sea overnight to produce fresh water and other operations.

Japan has evacuated hundreds of its citizens from Wuhan since the outbreak began. They have been asked to self-quarantine, with most staying in government-appointed hotels for a period of 14 days.

Evacuees from the first flight, which landed Jan 29, are expected to be released from self-quarantine this week after undergoing an additional test for the virus, officials said.

Japan is planning to send a fifth evacuation flight to Wuhan later this week.