'We suspect that it was bird flu that caused her death, but we cannot be 100 per cent sure because there was no sample for tests. At the time we thought her illness was something else,' Truong Thi Mau, director of the Ba Thuoc district hospital, told Reuters.

Ba Thuoc is about 150 kilometres south of Hanoi in Thanh Hoa province. Earlier this week, authorities confirmed that the 13-year-old girl's younger sister was in hospital with an H5N1 virus infection.

Both girls had eaten duck and chicken raised on her family's farm, Mau said.

The 13-year-old girl developed high fever and severe cough on Dec 25 and died in the district hospital on Jan. 2, Mau said. Her 8-year-old sister was still in hospital.

Five Vietnamese people died of bird flu in 2008 out of six reported H5N1 infections, all in the north of the country in the first quarter of the year, when cool temperatures allow the virus to thrive best.

The H5N1 strain has killed 247 people globally among the 391 confirmed cases of infection since 2003, according to the World Health Organisation (WHO).

Before the latest case in Vietnam, the WHO said Vietnam had had 106 human infections, the second highest number among 15 countries with known human cases after Indonesia. -- REUTERS