TOKYO (Reuters) - Japan has confirmed its first case of infection from a newly-identified coronavirus that has killed one person and infected at least 40 more in China, as well as prompting a travel alert from the U.S. State Department.

A man in his 30s from Kanagawa prefecture, next to Tokyo, tested positive, the health ministry said on Thursday. The man had been to the Chinese city of Wuhan, where there has been an outbreak of pneumonia believed to be caused by the new type of coronavirus, a ministry statement said.

The U.S. State Department issued a health alert update on Wednesday about travel to the Wuhan region. It referred to a Watch Level 1 Alert by the U.S. Centers for Disease Control and Prevention urging citizens travelling in the region to avoid contact with animals, animal markets or animal products, among other precautions.

Thai health authorities said on Wednesday they were stepping up monitoring of passengers arriving at airports ahead of the Lunar New Year holiday, when 800,000 Chinese tourists are expected to visit Thailand.

Coronaviruses are a large family of viruses that can cause infections ranging from the common cold to Severe Acute Respiratory Syndrome (SARS). Some of the virus types cause less serious disease, while some - such as the one behind Middle East Respiratory Syndrome (MERS) - are far more severe.

The World Health Organization (WHO) has said the new Wuhan virus could spread and has warned hospitals worldwide. [L8N29J2O2]

Jeremy Farrar, a British-based infectious disease specialist and director of the Wellcome Trust global health charity, said news of a case in Japan was “extremely concerning”.

He warned it was possible that multiple cases with mild symptoms might be masking the true numbers of people infected, indicating that many cases may be going unreported.

“Wuhan is a major hub and with travel being a huge part of the fast-approaching Chinese New Year, the concern level must remain high,” he said in a comment emailed to Reuters.

“It is probable that we are looking at patients being affected over a number of days from multiple animal sources and with some degree of human-to-human transmission.” A Wuhan seafood market in particular is being investigated in connection with the infection, authorities there said on Dec. 31.

The Japanese patient returned from Wuhan this month with a fever and was hospitalised. He was released on Wednesday after the symptoms subsided, the Health Ministry statement said.