Japan's Prime Minister Shinzo Abe has ordered a fresh check on the condition of Japanese passengers who got off the quarantined Diamond Princess cruise ship after one of them tested positive for the coronavirus days after leaving the ship.

"The situation has entered a crucial phase with cases with unknown infection routes being reported in multiple regions," Abe told his subordinates, adding that reliable and effective preventive measures must be implemented to curb the increase in patient numbers and contain the outbreak.

Abe's orders came as Japan's Emperor Naruhito－who turned 60 on Sunday－offered his sympathies to those affected by the outbreak.

Sunday marked Naruhito's first birthday since becoming emperor and is also considered a landmark age for people in both Chinese and Japanese cultures. However, festivities, including a large gathering were either canceled or toned down due to concerns over the virus.

Speaking at a news conference ahead of his birthday, Naruhito said: "As we face concerns about the spread of the coronavirus, I offer my sympathies to the patients and their families.".

"I hope that the escalation of the infections will be contained as soon as possible," he added.

The emperor's words came as infections in Japan were rising.

As of Sunday, the cases had risen to more than 838, including 691 from the Diamond Princess.

The cruise ship has been quarantined since Feb 5 with 3,700 people initially on board.

On Saturday, a woman in her 60s from Tochigi prefecture, north of Tokyo, who was on the ship with her husband, was confirmed to be infected.

"The woman had tested negative for the virus on Feb 15, four days before she was allowed to get off the ship with her husband, but she started to have a fever on Friday and tested positive on Saturday," a local official told China Daily in a telephone interview, adding that she and her husband had take a train home.

With the first passenger from the ship testing positive in Japan after disembarking from the vessel, the case has sparked questions about the effectiveness of the quarantine measures taken by the Japanese government.

"There was an assessment that those who had disembarked after testing negative had no problem, but it has now become clear that those people could be infected," Tochigi Governor Tomokazu Fukuda said at a press conference.

"So, I would like to urge the government to take more thorough preventive measures," he added.

In response, Japan's health minister, Katsunobu Kato, said the conditions for leaving the ship had been set according to experts' views, but the ministry will take the latest development seriously.

There are many things about the new virus that we are yet to learn, Kato said, adding that Japan would keep a check on the health of former passengers and tell them to stay at home as much as possible for at least two weeks.

Separately, on Saturday, Japan also said that 23 passengers who had been released from the Diamond Princess, were not tested for the virus due to procedural mistakes.

"We deeply regret that there was an operational error," Kato said at a news conference. "We will examine what went wrong so we do not repeat the mistake."

According to the ministry, the 23 were tested before the quarantine began on Feb 5, but they were allowed to leave the ship on Wednesday and Thursday without being tested again.

The viral outbreak has infected more than 78,000 people globally, according to the latest figures reported by each government's health authority as of Sunday in Beijing.

In a related development, South Korea's President Moon Jae-in put his country on its highest alert for infectious diseases, saying on Sunday that "unprecedented, powerful "steps would be taken to fight the outbreak as 123 more cases were reported on Sunday, raising the nation's total to 556 with five deaths.

Iran's health ministry raised Sunday the death toll from the new virus to 8 people in the country, amid concerns that clusters there, as well as in Italy and South Korea, could signal a serious new stage in its global spread. There were now 43 confirmed cases of the illness in Iran.