Two elderly Vietnamese people who are related to victims of bird flu have themselves contracted the disease, according to local health officials.
One of the new victims is the grandfather of two bird flu patients
The new cases, neither of which had shown any symptoms, will add to concerns the disease may be spreading more readily between humans.
Experts are worried the virus could eventually combine with human flu and risk a deadly pandemic.
Bird flu has killed at least 46 people in South East Asia since December 2003.
"We are aware of these [new] cases and we are investigating this further," a World Health Organization spokeswoman said.
One of the new cases, a 61-year-old woman whose husband died of bird flu, had no other known contact with the disease.
"She said she ate only pork and all four chickens raised in her house tested negative for bird flu," an official at the health clinic in her village in Thai Binh province told Reuters.
The other case, an 81-year-old man, is the grandfather of two teenage siblings who also have the disease.
H5N1 BIRD FLU VIRUS
Principally an avian disease, first seen in humans in Hong Kong, 1997
Almost all human cases thought to be contracted from birds
Isolated cases of human-to-human transmission in Hong Kong and Vietnam
It was not clear if he caught the disease from contact with infected poultry or with his grandchildren.
A 26-year-old nurse who looked after one of the teenagers is also now sick with bird flu.
The fact that the widow and the grandfather are apparently healthy, despite testing positive for the disease, suggests that there could be more bird flu cases than previously thought.
This concern was also raised on Tuesday, when the WHO announced that seven Vietnamese who initially tested negative for bird flu had now been found to have carried the virus.
"There's no doubt. The WHO accepts that we are missing cases. It's quite possible that some people are falling sick and their symptoms are very light and they don't end up in hospital," said WHO regional spokesman Peter Cordingley.
The World Health Organization warned last month of "the gravest possible danger of a pandemic".
During an international conference on bird flu in Ho Chi Minh City, Vietnam promised to overhaul its poultry industry, as part of efforts to stamp out bird flu.