International experts probing China's deadly H7N9 bird flu virus said Wednesday it was "one of the most lethal influenza viruses" seen so far as Taiwan reported the first case outside the mainland.

China has confirmed 108 cases and 22 deaths since the first infections were announced on March 31 and Taiwan Wednesday 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 Organization, which has led a team on a week-long visit to China to study H7N9.

Fukuda told a news conference that the H7N9 virus 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 were the likely source of the H7N9 outbreak as chickens, ducks and pigeons from markets had tested positive, but nevertheless warned over the potential for human-to-human transmission.

"So far no migratory birds or their habitats have tested positive for H7N9," said team member Nancy Cox, director of the influenza division at the US Centers for Disease Control and Prevention.

"At least we can now understand that the likely source of infection is poultry, the virus originates from poultry," she said.

Experts have previously said the animal reservoir for H7N9 appeared to be unspecified birds.

There are worries over the prospect of such a virus mutating into a form easily transmissible between humans, which could then have the potential to trigger a pandemic.

But a statement released by the team, which includes Chinese experts, repeated that no human-to-human transmission has been discovered.

"No sustained person to person transmission has been found," it said. "What remains unclear is whether the virus could gain the ability to become transmissible between people."

A WHO official said last week more than 50 percent of those with the virus had remembered coming into contact with birds, raising questions over how the remaining cases became infected.

Chinese health officials have acknowledged so-called "family clusters", where members of a single family have become infected, but have so far declined to put it down to human-to-human transmission.

Such cases could be examples of what health officials call limited human-to-human transmission, in which those in close contact with the ill become infected, as opposed to widespread, or "sustained", transmission.

So far most H7N9 cases have been confined to the commercial hub Shanghai and nearby provinces in eastern China.

But the number of reported new cases in Shanghai has seen a "dramatic slowdown", Cox said, calling the reduction in the frequency of new cases "very encouraging".

Tuesday marked the fourth consecutive day where no new cases were reported in Shanghai.