HANOI, Vietnam (AP): Bird flu killed a woman from northern Vietnam in the country's 51st fatality from the disease, and health officials fretted Tuesday that the virus would spread further.

The 23-year-old teacher from Phu Tho province, some 80 kilometers northwest of Hanoi, died in a Hanoi hospital Monday morning, said Nguyen Huy Nga, director of the Health Ministry's Preventive Medicine Department.

Test results showed that she was infected with the H5N1 bird flu virus. It was the country's 51st bird flu death reported since the virus began raging across Asia in late 2003, and the 233rd death reported globally.

``We still report bird flu outbreaks among poultry,'' Nga said Tuesday. ``The risk of bird flu infection among people remains very high and we expect more human cases.''

The woman developed bird flu symptoms on Feb.14 and died Monday after being treated at the National Tropical Disease Hospital for four days, Nga said.

Nguyen Duc Mao, director of Phu Tho's Preventive Medicine Bureau, said health authorities are still unclear how the woman got infected.

``A few hundred chickens died in the school neighborhood, but they tested negative for the bird flu virus,'' Mao said. ``We still don't know how she was infected.''