HANOI, May 25 (Reuters) - Bird flu has struck poultry in two more places in northern Vietnam, officials said on Friday, two days after reporting the first human case in a year and a half.

Tests on ducks and chickens at poultry farms in the port city of Haiphong and in Bac Giang province showed they were infected with the H5N1 virus, the Animal Health Department said.

The department also said there had been fresh outbreaks in the previously hit northern provinces of Quang Ninh and Nam Dinh, killing ducks and chickens that had not been vaccinated.

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 on Wednesday it recorded the first human case since November 2005 when a man from Vinh Phuc province near the capital, Hanoi, was found to have contracted H5N1 after he helped slaughter chickens at a friend"e;s wedding.

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 for all poultry.

The H5N1 virus has killed 42 people in the Southeast Asian country since it resurfaced in Asia in late 2003.

The virus returned to poultry in the south late last year and earlier this year. Last month, the WHO urged Vietnam to accelerate poultry vaccination and target more ducks.

Vietnam has so far this year vaccinated 120.8 million poultry in 60 out of its 64 provinces, officials said.

Apart from killing birds that have not been vaccinated, the spread of the virus in May at the beginning of summer is unusual because experts had previously said it thrived best in cool temperatures and weakened in warmer weather.