Image caption Taiwan is stepping up prevention measures against bird flu

A 53-year-old businessman in Taiwan has the first case of the H7N9 bird flu virus outside mainland China, health officials there have confirmed.

The man is in a serious condition in hospital days after returning from the Chinese city of Suzhou, officials say.

China has confirmed 108 cases of H7N9 since it was initially reported in March, with at least 22 people dead.

The World Health Organization (WHO) says this strain appears to spread more easily from birds to humans.

The man in Taiwan was brought to hospital three days after he arrived from Suzhou via Shanghai, officials say.

He was not in contact with poultry, nor had he eaten undercooked birds while in Suzhou, Taiwanese Health Minister Chiu Wen-ta told local media.

Taiwan's President Ma Ying-jeou has ordered the health department to step up prevention measures, says the country's Central News Agency.

'Unusually dangerous'

Experts are still trying to understand the H7N9 virus, and it has not yet been determined whether it could be transferred between humans.

"This is definitely one of the most lethal influenza viruses we have seen so far," WHO flu expert Dr Keiji Fukuda said at a news conference in Beijing.

"When we look at influenza viruses this is an unusually dangerous virus."

He added that the WHO team was just beginning its investigation. But he said that based on the evidence, "this virus is more easily transmissible from poultry to humans than H5N1", a strain which spread in 2003.

Dr Fukuda led a team from the WHO on a one-week China visit to study H7N9, along with Chinese officials from Beijing and Shanghai.

The WHO believes that poultry is still the likely source of the H7N9 outbreak in China.