The Diamond Princess, which is still carrying about 3,500 passengers and crew members, now has 218 people infected with the virus out of 713 people tested since it entered Yokohama Port on Feb. 3, the largest cluster of infections outside China.

In all, Japan has 247 confirmed cases of the new disease that apparently started in Wuhan, a city in central China, in December.

Health Minister Katsunobu Kato said five of the patients sent to hospitals earlier have severe symptoms and are on artificial respirators or under intensive care.

Kato also said the government has decided to allow passengers older than 80 to get off the ship after testing negative for the virus. He said results of tests on about 200 eligible passengers are underway, and those with chronic health problems or in cabins without operable windows will be given priority.

Kato said the measure is to reduce health risks for passengers stuck in rooms under difficult conditions. Those who are released will be asked to stay at a designated facility through the end of the quarantine period.

"We are doing our utmost for the health of crew members and passengers who remain on the ship," Kato told a news conference.

Some experts have questioned Japan's strategy of isolating the passengers and crew in a potentially virus-affected environment on the ship while the disease is already slowly making its way into the country.

"On the ship, infections are getting very dense," said Shigeru Omi, an infectious disease prevention expert and former regional director for the World Health Organization. "It's like we are seeing a very condensed version of what could happen in a local community."

Omi, who currently heads the Japan Community Health Care Organization, said those people who have tested positive for the virus are "only a fraction" of what could already be spreading outside of the ship. "We should assume that the virus has already been spreading in Japan," he said.