Vietnamese authorities say the H5N1 strain of bird flu has struck farms in four provinces in Vietnam, killing almost 2,000 unvaccinated ducks.
Authorities said Tuesday the virus has been found in the three northern provinces of Son La, Quang Ninh, Nam Dinh and in the country's southern Mekong Delta province of Can Tho.
Nearly 6,000 birds have been culled.
Earlier this month a bird flu outbreak was reported in Vietnam's central province of Nghe An.
Vietnam has been praised for its efforts to contain the spread of the virus through culls, vaccinations and public education.
Vietnam was once the country with the greatest number of human deaths from bird flu. However, it has not had any human fatalities since late 2005. Indonesia now has the most human deaths with 76.
The H5N1 strain of bird flu has killed at least 185 people since late 2003, mostly in Southeast Asia.
The virus is usually passed to humans through infected animals, but experts fear it could mutate into a form easily passed between people.
Some information for this report was provided by AFP, AP and Reuters.