Eleven people have now died of 15 confirmed cases in Vietnam, which has suffered the worst human impact of bird flu.
The remaining four deaths have been in Thailand, the only other nation to have reported human fatalities from the disease.
The latest victims were a 24-year-old man from central Lam Dong province, who died on Tuesday, and a 15-year-old girl from Tay Ninh in the south, who passed away on 27 January, Vietnamese health officials said.
"Both victims tested positive for the highly pathogenic H5N1 strain of avian influenza at the city's Pasteur Institute," Ho Chi Minh City's health department deputy director Truong Trong Hoang said.
Ten countries in Asia have reported outbreaks of bird flu, and WHO warned on Wednesday the epidemic was set to hit other countries in the region.
"The speed with which the virus is spreading suggests that nowhere in the region is safe. The virus is spreading faster than we can get to it"
"The speed with which the virus is spreading suggests that nowhere in the region is safe," a spokesman for WHO's Manila-based Western Pacific office, Peter Cordingley, said.
"The virus is spreading faster than we can get to it."
Bird flu has so far emerged in Cambodia, China, Indonesia, Japan, Laos, South Korea, Thailand and Vietnam, while Taiwan and Pakistan have reported weaker strains.
China on guard
Meanwhile, the Chinese government on Wednesday demanded a more comprehensive approach from its regions to curb bird flu, as it spread rapidly across the country.
Ten countries across Asia have
reported bird flu outbreaks
"One of the tasks is to take immediate, effective surveillance, quarantine and disinfection steps to stop the epidemic spreading," Vice-Premier Hui Liangyu said at a meeting of the National Bird Flu Prevention Headquarters.
Hui, cited by the Xinhua news agency, said better communication and cooperation across China, where farm methods are often primitive and corruption is rife, was needed to tackle the crisis.
There are 21 bird flu outbreaks affecting 12 provinces and municipalities in China, which is the world's second-largest producer of chicken meat and the fifth-largest exporter.
China, along with Vietnam, Thailand and Indonesia, are among the Asian nations accused of covering up bird flu last year when a speedy response may have headed off the emerging crisis.
WHO's Cordingley said bird flu might have emerged "as far back as the middle of last year".
"We think the virus has been around a lot longer than initially thought. It's quite well embedded in some areas," he said.
But while Cordingley warned the disease was spreading quickly across Asia, he gave an ambiguous response on whether it would affect other parts of the world.
"We have no idea whether it could spread beyond Asia," Cordingley said. "The chances of it showing up in Europe for example, are very slight."
The continued culling of birds, the ban on imports of poultry from affected areas and improved hygiene measures would help control the spread, he said.
More than 25 million chickens have been culled across Asia, with the disease believed to have been passed on to humans so far only through direct contact with sick birds or their droppings.