(Updates with agriculture ministry)

By Ho Binh Minh

HANOI, Oct 11 (Reuters) - Bird flu returned to southern Vietnam this week after an absence of two months, and officials warned farmers of more outbreaks as the weather cools.

Tests performed at a laboratory for the Mekong delta region confirmed the H5N1 virus in the samples taken from ducks at a farm in Tra Vinh province, the Agriculture Ministry said in a report on Thursday.

The infection emerged last week when five ducks died among 300 at the farm. None of the ducks had been vaccinated against the virus, the report said.

Animal health workers have since slaughtered the remaining ducks.

A Tra Vinh Animal Health Department official said by telephone that the last bird flu outbreak was found among ducks in the same district in late August.

Agriculture Minister Cao Duc Phat on Tuesday urged animal health authorities to step up vaccinating poultry because bird flu would soon return among unvaccinated birds, especially as the weather cooled in autumn and winter in northern provinces.

The second phase of vaccination is underway in 23 out of Vietnam"e;s 64 provinces and 38.8 million birds have been injected.

Bird flu has infected seven people in Vietnam so far this year, four of whom have died, bringing the death toll since late 2003 to 46.

Globally, the H5N1 virus has killed 202 people out of 330 known cases, according to the World Health Organisation.

Hundreds of millions of birds have died or been slaughtered.

But Vietnam"e;s poultry stock has been increasing, with 226 million birds reported at the end of August, up 5.3 percent from a year ago, of which the waterfowl stock expanded 8.7 percent to 68 million, government figures showed.