HANOI, May 28 (Reuters) - Bird flu has spread to another province in northern Vietnam and now nine cities and provinces have been struck by the deadly virus in less than a month, the Agriculture Ministry said.

Tests on ducks at a poultry farm in Ninh Binh province showed they were infected with the H5N1 virus, the Agriculture Ministry"e;s Animal Health Department said in a report seen on Monday.

About 1,200 of the 1,750 ducks on the farm had been vaccinated against the H5N1 virus, the report said.

It also said no fresh outbreaks were reported in the other eight cities and provinces in the past few days.

International public health authorities describe Vietnam"e;s mass poultry vaccination programme and other measures as a model for keeping 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"e;s wedding about a month ago.

The World Health Organisation 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 was not alarming 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, 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.