TOKYO (AFP) -- An additional 39 people on board the Diamond Princess cruise ship off the Japan coast have tested positive for the new coronavirus, Japanese Health Minister Katsunobu Kato said Wednesday, bringing the total to 174.

"Out of 53 new test results, 39 people were found positive," he told reporters, adding that a quarantine official had also been infected with the virus.

He added that: "At this point, we have confirmed that four people, among those who are hospitalised, are in a serious condition, either on a ventilator or in an intensive care unit."

The Diamond Princess has been in quarantine since arriving off the Japanese coast early last week after the virus was detected in a former passenger who got off the ship last month in Hong Kong.

When the boat arrived off Japan, authorities initially tested nearly 300 people of the 3,711 aboard for the virus, gradually evacuating dozens who were infected to local medical facilities.

In recent days, testing has expanded to those with new symptoms or who had close contact with other infected passengers or crew.

Those who remain on the ship have been asked to stay inside their cabins and allowed only briefly onto open decks.

They have been asked to wear masks and keep a distance from each other when outside, and given thermometers to regularly monitor their temperatures.

The ship is expected to stay in quarantine until February 19 -- 14 days after the isolation period began. (AFP)