Vietnamese animal health workers have detected bird flu in a flock of ducks in the southern Mekong Delta, officials have said amid fears of a wider resurgence of the virus.

Four ducks from two farms were found to carry the H5 virus in laboratory tests carried out by the Ho Chi Minh City regional Animal Health Centre, said an official with the Ben Tre province Animal Health Department.

"We took random samples for tests," said the official. "The ducks were neither ill nor dead. We are now taking more samples for more tests."

An expert at the Ho Chi Minh City lab said "the samples tested positive for the H5 bird flu virus".

"Vietnam cannot make the test for the N component itself," he said. "In many cases we have to send the samples for tests to Britain, the United States or Australia."

The UN Food and Agriculture Organisation´s technical advisor on bird flu in Vietnam, Jeff Gilbert, said "testing for the N (component) is more expensive and technically more difficult.

"In a case like Vietnam, which has been previously infected, you would take the assumption it is H5N1 and immediately take the appropriate response without waiting for the final results."

Vietnam, the country worst hit by H5N1 after Indonesia with 42 human deaths since late 2003, has reported no new human cases this year.

However, experts have warned the virus remains widespread in Vietnam, including in ducks that can carry the virus without showing signs of illness.

Last week authorities killed more than 50 storks after several of them were found to carry the H5 virus in an amusement park outside Ho Chi Minh City, the former Saigon.

Prime Minister Nguyen Tan Dung on Tuesday ordered officials to remain highly vigilant after Thailand suffered two human deaths from the H5N1 strain and Laos saw an outbreak on a chicken farm in recent weeks.

Gilbert praised Vietnam for its speedy reporting of the latest cases but warned that "it is very important to keep vaccinating and monitoring the outbreaks to be able to quickly take the appropriate measures".

"There is no surprise it´s coming back," he told AFP. "We need to improve the surveillance network. With an early warning system ... you have much better control."