HANOI, May 6 (Reuters) - Bird flu has been found on a duck farm in central Vietnam, the first outbreak of the disease in more than a month, the government said on Sunday.

Tests showed the H5N1 virus had killed 160 ducklings in the farm in Nghe An province on May 1, the Agriculture Ministry"e;s Animal Health Department said in a report. It said the 40-day-old ducklings had not been vaccinated against bird flu.

By Friday another 90 ducks died in the same farm, prompting health workers to slaughter the remaining 360 fowl, the report said.

The H5N1 virus has killed 42 people in the Southeast Asian country since it re-surfaced in Asia in late 2003 but Vietnam has had no human cases since November 2005.

The virus flared up again among poultry in the south late last year and earlier this year.

Last month the World Health Organisation (WHO) urged Vietnam to accelerate poultry vaccinations and target more ducks in its anti-bird flu campaign.

Waterfowl are a reservoir for the disease and can spread the H5N1 virus in their droppings as they roam through rice fields. Ducks often show no symptoms of sickness, making it harder to contain the virus.

The Animal Health Department said 60 of Vietnam"e;s 64 provinces have so far finished or nearly completed the first of a two-phase vaccination campaign which targets up to 90 percent of the country"e;s poultry stock.

The virus has killed 170 people in 11 countries, most of them in Indonesia and Vietnam, according to the WHO.