The 14-year-old boy died four days after being admitted to hospital for treatment with flu-like symptoms.
Fears are mounting across the region after China reported its first human case in months and Vietnam had its first outbreak in poultry in a year.
There is still no evidence the virus can be passed easily between humans.
Doctors said chickens had recently died in the area where the boy lived, suggesting he may have come into contact with infected poultry.
On Wednesday, China also reported its first case of human bird flu in six months, but said the man in question has since made a full recovery.
But there has been no reported outbreak of the virus among poultry in the area where the farmer lived, leading to questions as to how he contracted the disease and raising fears it is circulating undetected among birds.
"In China, the challenge is now to identify where this virus is hiding and how it is circulating," Henk Bekedam, the World Health Organization's China representative, told Reuters.
Vietnam meanwhile has slaughtered thousands of birds following a fresh outbreak of the disease in December.
The country had been free of human cases of bird flu for a year - thanks to a mass culling and vaccination drive.
However, as temperatures drop for the Northern hemisphere's winter - when the virus survives longer - experts say there is a heightened risk of the disease spreading to humans.
According to the WHO, the disease has claimed more than 150 lives since it began ravaging Asian poultry farms in late 2003, and triggered the mass slaughter of millions of birds.
There are fears the virus could mutate into a form that is easily passed between humans, but so far the vast majority of cases can be traced back to contact with infected birds.Güncelleme Tarihi: 20 Eylül 2018, 18:16