HANOI (AFP) - The World Health Organisation is investigating a suspected human case of bird flu in Vietnam, a spokeswoman said Wednesday following a series of new outbreaks on poultry farms.

If confirmed, it would be the first human infection in one-and-a-half years in Vietnam, one of the countries hardest hit by the virus with 42 human fatalities between 2003 and November 2005.

"We're working closely with the ministry of health on a suspected H5N1 case," WHO spokeswoman Dida Connor told AFP.

"The virus has been circulating for a long time in the poultry. If this was to be a human case, it would not be surprising."

Vietnamese state media have reported that an initial test last Sunday on a 30-year-old farmer from northern Vinh Phuc province, now in a critical condition in a Hanoi hospital, had proved positive for bird flu.

The man had helped slaughter chicken for a wedding about one month ago from a neighbouring farm, where several of the flock of 500 birds later died. A Vietnamese veterinary team this week went to assess the farm.

Connor stressed that the WHO was awaiting Vietnamese test results on the case, which would then have to be verified by the UN health body's own laboratory tests.

"We don't yet have an official confirmation of this suspected case, but we are following it closely," health ministry spokesman Nguyen Duc Long said.

Vietnam has reported no new human infections since November 2005.

The virus, however, remains endemic in wild and domestic bird populations and has struck again this month with seven outbreaks of the H5N1 strain in five provinces killing thousands of unvaccinated ducks.

The affected farms stretch from Son La province in the mountainous north to the southern Mekong Delta province of Can Tho, said the animal health department in an online report.

Avian influenza is known to have killed 185 people worldwide since late 2003, most of them in Southeast Asia, according to WHO figures.

In its present form, H5N1 is lethal for birds and people in close proximity to infected fowl. Experts fear that it could one day mutate to easily spread from human to human and trigger a deadly pandemic.