Bui Quang Anh, head of the Agriculture and Rural Development Ministry's Veterinary Department made the revelation, adding that bird flu development in the country is becoming more and more complicated.

If no drastic measures are to be taken, bird flu will come back to any localities, Mr Anh warned.

Of random poultry serum samples collected in the Mekong Delta provinces, four from Bac Lieu and 33 others from Dong Thap province have tested positive for bird flu virus strain H5N1.

Tested serum samples of ducks and white-winged ducks in the northern provinces of Ninh Binh, Ha Nam and Vinh Phuc also showed the same results.