Viruses in both the chickens and the people appear to be similar - but further testing is being done.
"There is a possible link," said Peter Horby, a WHO epidemiologist in Hanoi. "In the past, chickens have infected people in outbreaks in Hong Kong in 1997 and again in 2003."
Horby said he's expected to meet with the Ministry of Health on Tuesday to further discuss the possible connection, but he's optimistic it can be contained.
"Whatever they've got, there doesn't seem to be widespread transmission," he said. "It's encouraging that's it's not spreading."
In addition to the 11 deaths, three others are sick with the virus. All the cases are in Hanoi.
Horby said all the victims are believed to have come into contact with poultry before falling ill.
Vietnamese scientists are working to determine whether the disease can jump to humans as it did in 1997 in Hong Kong when six people died from the H5N1-97 strain, said Dau Ngoc Hao, deputy director of the Veterinary Department in Hanoi.
Most of the sick chickens have surfaced in the southern Mekong Delta provinces of Long An and Tien Giang where officials continued a cull. Last week, 81,000 chickens also were buried at a Thai-owned chicken farm in northern Ha Tay province, some 25 miles west of Hanoi. Almost 600,000 chickens have fallen ill countrywide.
The virus, which is mutating quickly, has also begun spreading to other poultry such as ducks and geese, said Hao.
He said there were no reports of humans sickened by a similar virus in the southern provinces.
Samples of the bird flu have been sent to the U.S. Centers for Disease Control and Prevention, but results were not expected until next week.
About 1.1 million chickens and ducks were culled last month in South Korea to contain a bird flu that broke out there, and officials in Japan on Monday said 6,000 chickens had died there and thousands of others would be slaughtered.
By Margie Mason