The death toll from the H7N9 bird flu virus has risen to 27, state media said Thursday after a man died in central China's Hunan Province.

The 55-year-old whose surname was given as Jiao died on Wednesday after receiving medical treatment, state news agency Xinhua said, citing local authorities.

More than 120 people have been diagnosed with the virus since it was first reported in late March, with most cases confined to eastern China.

The only one reported outside the mainland has been in Taiwan. That victim was infected in China, but led to Asian countries urging renewed vigilance against the virus.

Experts fear the possibility of the virus mutating into a form easily transmissible between humans, with the potential to trigger a pandemic.

The World Health Organization has said so far there is no evidence of human-to-human transmission but warned H7N9 is "one of the most lethal" influenza viruses ever seen, and urged travellers against contact with live poultry.

Chinese researchers, reporting in The Lancet a week ago, said they had confirmed poultry as a source of the virus.

Chinese health officials have acknowledged so-called "family clusters", where members of a single family have become infected, but have not established any confirmed instances of human-to-human transmission.

Most of the cases reported have not yet resulted in death, and some patients have been discharged from hospital after apparently recovering.

China confirmed 19 new cases of the virus in the week leading up to May 1, Xinhua said.

But the number new cases in Shanghai has seen a "dramatic slowdown", Nancy Cox, director of the influenza division at the US Centers for Disease Control and Prevention said last week, describing the slowdown as "very encouraging".