TOKYO - Japan confirmed its first case of infection from the mystery Wuhan pneumonia-like virus on Thursday (Jan 16), as Vietnam said it has isolated two visitors from the Chinese city as a preventive measure.

Japanese health authorities said a resident in his 30s of Kanagawa prefecture, south of Tokyo, had tested positive for the new virus strain. But officials took pains to stress that this was an isolated case, and that nobody else in Japan are suspected to have been infected.

"It has not been confirmed at this moment that sustained human-to-human infections can occur," Japanese Chief Cabinet Secretary Yoshihide Suga said.

The man, a Chinese national, had been in close contact with a patient when he was in Wuhan, the Health Ministry said. It added that none of his family members who live with him in Japan, nor the doctors who treated him, have tested positive for the virus.

The Wuhan outbreak coincides with the annual flu season in Japan, and the ministry has reiterated its advisory for people to wash their hands, gargle, and wear masks to avoid falling sick.

Meanwhile, two Chinese tourists with fever symptoms from Wuhan were quarantined by Vietnam's Ministry of Health and are now under observation after they landed at Danang International Airport on Tuesday (Jan 14). Hanoi dispatched a medical team to the coastal city and called an emergency meeting on Wednesday on prevention efforts.

Japan's is the second confirmed Wuhan virus case to be reported outside China, after Thailand confirmed on Monday (Jan 13) that a 61-year-old Chinese woman had been hospitalised last week for high fever and breathing difficulties. She is now in a stable condition.

The Kanagawa resident had first developed fever symptoms on Jan 3 when he was in Wuhan. He returned to Japan on Jan 6 but was not detected at the immigration health checkpoint as his fever had been brought down by medication.

He saw a doctor later the same day, to whom he reported his travel history to Wuhan, but was prescribed medication and could return home as his symptoms were relatively minor.

He went to a hospital on Jan 10 as his fever persisted and was hospitalised until Wednesday (Jan 15), when he was discharged. He does not have a fever but reportedly still has a slight cough.

Later that evening, Japan's National Institute of Infectious Diseases, which prepared diagnostic kits after China shared the genetic sequence of the coronavirus on Sunday, confirmed that the man had been infected with the new coronavirus strain.

When asked why the announcement was not made upon the discovery, Mr Suga said that it was a conscious decision given that the situation was contained.

There has been growing regional concern over the outbreak of pneumonia in Wuhan that has been linked to a new coronavirus strain.

The family of pathogens is behind diseases ranging in severity from the common cold to the severe acute respiratory syndrome (Sars), which killed hundreds in Asia in 2002 and 2003.

Coronaviruses cause symptoms typical of the common cold, by infecting the nose, sinuses or upper throat. They are spread through sneezing, coughing, or direct contact.

Many of the 41 reported cases in Wuhan were traced to the Wuhan Huanan Seafood Wholesale Market. But neither patient in the Thai nor Japanese case had visited the market, which has been closed since Jan 1 to contain the outbreak.

No new cases have been reported in Wuhan since Jan 3. One has died, while at least seven patients were discharged.

The World Health Organisation, which has said that the virus has "some limited human-to-human transmission", is not taking any chances and has warned of a potential wider outbreak.

The region is gearing up on precautionary measures with an expected surge in Chinese visitors during the Lunar New Year holiday later this month, with more stringent health monitoring checks and quarantine procedures for those who have visited Wuhan recently.

Many countries in the region, including Japan and Indonesia, are stepping up health screening of inbound travellers.

The Japanese government has called on hospitals to swiftly report any cases of patients with a recent travel history to Wuhan, while also setting up an information liaison office at its crisis management centre to monitor and respond to any new cases.