The Animal Health Department said in a report on Sunday the H5N1 bird flu virus had been found in ducks in the Mekong Delta province of Tra Vinh, extending the spread of bird flu in southern provinces in recent weeks.

Tra Vinh is adjacent to Vinh Long province where bird flu struck a chicken farm a week ago.

Officials have confirmed outbreaks of the virus in ducks and chickens in four other Mekong delta provinces further southwest of Ho Chi Minh City, where market inspectors have restricted the movement and sale of poultry.

Vietnam has had no human H5N1 cases since November 2005 but the virus that first hit the Southeast Asian country in late 2003 re-emerged last month in Mekong delta poultry.

Agriculture officials have warned the country's 84 million people that the virus could spread nationwide via migrating birds.

Bird flu killed 42 of the 93 people infected in Vietnam in 2003-2005.

In Japan, agricultural authorities began culling thousands of chickens at a southern Japanese poultry farm Sunday after test results found a pathogenic strain of the bird flu virus, officials said amid the nation's latest outbreak of the disease.

Some 4,000 chickens have died at a farm in the town of Kiyotake in Japan's southern prefecture (state) of Miyazaki since last week, and local officials said late Saturday that the virus belonged to the broad H5 family.

It was not yet clear whether the virus was the H5N1 strain that has killed dozens of people worldwide since 2003. But prefectural poultry official Keijiro Tarumizu said test results so far suggested that the virus was virulent enough for all the chickens at the farm to be destroyed.

On Sunday, about 150 local officials in white protective gear began culling the farm's surviving birds, packing them in plastic bags for fatal gassing before burying or burning them. Public broadcaster NHK showed the operation at the farm, where ground was covered with white disinfectant.

The H5 subtype is a highly pathogenic form of the virus among poultry, but is not necessarily fatal to humans.

Bird flu killed 42 of the 93 people infected in Vietnam in 2003-2005.

Since 2003, the H5N1 strain of the bird flu virus has claimed at least 157 lives worldwide, according to the WHO.