A passenger who got off the Diamond Princess after completing the cruise ship's quarantine this past week tested positive for the novel coronavirus on Saturday, becoming the first known case of infection among those released at the end of the ship's containment period, Japanese officials said.

The patient is a woman in her 60s from Tochigi prefecture, north of Tokyo, who was on the ship with her husband, Tochigi Gov. Tomikazu Fukuda said at a televised news conference.

The woman had tested negative for the virus on Feb. 15, four days before she got off the ship with her husband, with neither showing any symptoms at the time. The couple took a train home, officials said.

Despite strong doubts raised from inside and outside the country, Japanese health ministry officials have insisted that any passengers who completed the 14-day quarantine, tested negative for the virus and showed no symptoms had nearly zero risk of becoming a virus patient.

"I would like to urge the government to take more thorough preventive measures," Fukuda said sternly at the news conference.

Some experts and former passengers have criticised the quarantine, saying anti-infection measures were inadequate. The US, Australia and other governments that evacuated their citizens from the ship are requiring them an additional two-week quarantine.

Japan's health minister, Katsunobu Kato, said Saturday that 18 Americans, six Australians and one Israeli passenger who were flown out of Japan by their respective governments before the end of the ship''s quarantine tested positive after returning home. He said the results were understandable because those passengers did not fully meet the stricter requirements for release from the ship.

Kato said Japanese officials were working closely with their respective countries'' health officials to follow up the cases.

Kato also said Saturday that 23 passengers had been released from the Diamond Princess at the end of the quarantine without being tested for the virus due to procedural mistakes, another sign of sloppiness in the quarantine of the ship, where more than 600 people were infected.

He said the 23 were tested before the quarantine began February 5, but were allowed to leave the ship on Wednesday and Thursday without being tested again. Three of them have since tested negative, and most of the others have agreed to be tested, he said.

Kato said officials have tracked all 23 passengers down and asked them to self-quarantine at home for 14 days.

"We deeply regret that there was an operational error," he said at a news conference. "We will examine what went wrong so we will not repeat the same mistake." Japan has confirmed more than 760 cases of the coronavirus, which first emerged in China, including at least 634 from the Diamond Princess, which docked and was quarantined in Yokohama, near Tokyo.

The ship initially carried about 2,600 passengers and 1,100 crew members.

Most of the passengers were either taken to hospitals, disembarked or took chartered flights home. About 100 others who had infected roommates or contact with other passengers during the quarantine period were taken to a government facility to complete the 14-day quarantine.