Japanese public broadcaster NHK reported the passengers were a man and a woman in their 80s, citing an unidentified government source.

The latest numbers on infections in mainland China suggest the epidemic there is slowing, but it is unclear how much the fall in new cases is due to changes in diagnostic methodology or because fewer people are contracting the disease.

China's central Hubei province, the epicentre of the outbreak, reported 349 new confirmed cases, down from 1693 a day earlier and the lowest since January 25.

The death toll in the province rose by 108, bringing to total in China to over 2100 deaths and 74,500 cases.

Scientists in China who studied nose and throat swabs from 18 patients infected with the new coronavirus say it behaves much more like influenza than other closely related viruses, and could be passed on by people before they show symptoms.

The preliminary findings published on Wednesday in the New England Journal of Medicine offer evidence that the novel coronavirus is not like other coronaviruses such as Severe Acute Respiratory Syndrome (SARS).

"If confirmed, this is very important," said Dr. Gregory Poland, a sinologist and vaccine researcher with the Mayo Clinic in Rochester, Minnesota, who was not involved with the study.

In Japan, the government has come under intense criticism for its handling of an outbreak on a cruise ship carrying about 3700 people, that was placed in quarantine off Yokohama on February 3.

A second group of about 600 Japanese and foreign passengers from the Diamond Princess was set to disembark on Thursday, after the first batch were released from quarantine a day earlier.

Hundreds of people who left the chip on Wednesday were placed in quarantine again in their home countries, with Australia and Hong Kong putting their returned citizens in supervised isolation for another two weeks.

Even as the first passengers disembarked on Wednesday, Japanese authorities announced 79 new cases had been discovered on board, bringing the total to at least 620. That is over half of the known infections outside mainland China.

From the start, experts raised questions about quarantine on the ship.

Passengers were not confined to rooms until February 5. The day before, as passengers were being screened, ship events continued, including dances, quiz games and an exercise class.

The US Centers for Disease Control and Prevention (CDC) said Japan's efforts "may not have been sufficient to prevent transmission among individuals on the ship".

Japanese Health Minister Katsunobu Kato defended Tokyo's efforts.

"Unfortunately, cases of infection have emerged, but we have to the extent possible taken appropriate steps to prevent serious cases," Kato said in a report by state broadcaster NHK.

With China's Hubei province in virtual lockdown, strict limits on movement in several major Chinese cities, and bans on travel to and from the mainland, economists expect the world's second-biggest economy to take a short-term hit from the epidemic.

Financial leaders of the world's 20 largest economies (G20) said the coronavirus epidemic was a downside risk for the global economy, according to a draft communique prepared for a meeting on February 22-23.

Australian Associated Press