Taiwan has reported the first case of bird flu outside the Chinese mainland.
The news comes as international experts probing China's deadly H7N9 bird flu virus said it is "one of the most lethal influenza viruses" seen so far.
China has confirmed 108 cases and 22 deaths since the first infections were announced on March 31 and Taiwan has now confirmed its first infection in a man who had recently returned from working in eastern China where most cases have been reported.
"This is definitely one of the most lethal influenza viruses we have seen so far," said Keiji Fukuda, one of the leading flu experts for the World Health Organisation which has led a team on a week-long visit to China to study the virus.
Mr Fukuda told a news conference that the latest strain was more easily transmissible than the more common H5N1 strain of bird flu - experts had previously remarked on the "affinity" of H7N9 for humans.
"We think this virus is more transmissible to humans than H5N1," he said, referring to the strain the WHO estimates has killed more than 360 people globally since 2003.
"When we look at influenza viruses this is an unusually dangerous virus," he said, but he added: "We are really at the beginning of our understanding."
Taiwanese health authorities said their first case, a 53-year-old man who had been working in the eastern Chinese city of Suzhou, showed symptoms three days after returning to Taiwan via Shanghai.
Health Minister Chiu Wen-ta told reporters the patient said he had not been in contact with poultry or eaten under-cooked birds or eggs while staying in Suzhou.
The WTO team, however, said poultry was the likely source of the H7N9 outbreak as chickens, ducks and pigeons from markets had tested positive. Nevertheless, they warned over the potential for human-to-human transmission.
"No sustained person-to-person transmission has been found," a statement from the team said. "What remains unclear is whether the virus could gain the ability to become transmissible between people."
A WHO official said last week that more than 50% of those with the virus had remembered coming into contact with birds, raising questions over how the remaining cases became infected.