Some 250 fowl died Friday at a farm in Hoa Lu district of Ninh Binh province, 100 kilometers (62 miles) south of Hanoi, the department said on its website.

The rest of the almost 2,000-strong flock were culled immediately after test results showed they were carrying the H5N1 virus, it said.

Bird flu outbreaks have now been reported in provinces across Vietnam, with the north the worst affected.

Vietnam is one of the countries hardest hit by bird flu, with 42 human fatalities between 2003 and 2005.

Its world-acclaimed mass poultry vaccination program and other measures had helped keep the virus at bay for the past 18 months.

But last Wednesday, it recorded the first human case since November 2005 when a man from Vinh Phuc province near Hanoi was found to have contracted H5N1 after he helped slaughter chickens at a friend's wedding about a month ago.

The World Health Organization said it was working with the government to investigate the case and the WHO needed to verify the virus sample.

It said finding the suspected patient would not sound alarm bells if it was an isolated case.

The Animal Health Department said the Agriculture Ministry had ordered provincial animal health authorities to step up efforts to stamp out the disease, including swift anti-bird flu vaccination of all poultry.

The H5N1 virus has killed 42 people in Vietnam since it re-surfaced in Asia in late 2003.

The virus returned to poultry in the south late last year. Last month, the WHO urged Vietnam to accelerate poultry vaccination and target more ducks, which can carry the virus without showing symptoms.

Vietnam has vaccinated more than 120 million poultry in 60 of its 64 provinces so far this year, officials said.

The spread of the virus in May, at the beginning of summer, is unusual as experts say it appears to thrive best in cool temperatures and weakens in warmer weather.

Reported by Bui Tuan – Translated by The Vinh