Prolonged cold weather in Vietnam is causing more outbreaks of bird flu across the country after the disease killed two people last month, authorities warned on Monday.

“The weather this winter is much colder and longer than other winters which has led to outbreaks of the virus,” said Hoang Van Nam, director of the government’s animal health department.

At least four provinces across the country have reported re-occurrences of the H5N1 virus this year. About 4,000 birds have been destroyed, including 1,700 infected ones.

Many birds were transported across the country for the lunar New Year holiday without undergoing proper controls. The department said that the virus would continue to spread in the cold weather.

The government on Monday warned against buying poultry of unknown origin, and advised washing hands carefully after handling birds.

Last year, the virus killed 151,300 domestic fowl, 50 per cent fewer than 2010, authorities said in December. In 2010, seven people were infected with the virus, two of whom died.

There have been 121 confirmed human H5N1 infections and 61 deaths in Vietnam since the virus first appeared in the country in 2003.