A teenage boy in Vietnam and a woman in Thailand on Monday became the latest victims of the bird flu (search) sweeping Asia as world health authorities sought to confirm the first possible case of human-to-human transmission.

Bird flu has now killed 12 people in Asia and millions of chickens have been destroyed to prevent the spread of the virus.

The outbreak is "far from being under control," said He Changchui of the U.N. Food and Agriculture Organization. (search) "It remains a serious public health and animal threat, particularly in China, Thailand and Vietnam."

The latest victims were a teenage boy in Vietnam and a 58-year-old woman in Thailand who raised chickens. Those countries are the only ones where humans have died from this strain of avian influenza.

The World Health Organization (search) said there was no evidence this strain can be passed easily among people. The WHO continues to investigate a case in which the disease may have been transmitted between people, rather than from birds to people. If so, it would be the first recorded instance of human-to-human transmission in the current outbreak, raising the prospects of the virus mutating into a form that passes easily between people.

The case revolves around a wedding in Vietnam on Dec. 30 for which the groom and one of his sisters prepared a duck, WHO said. The groom fell ill Jan. 3. The sister who prepared the duck with him, as well as a second sister and the bride, got sick the following week. The groom and both sisters died, but the bride survived.

Investigators say they have found nothing to indicate the second sister or the bride were infected through exposure to poultry. That would suggest human-to-human transmission, but the WHO cautioned it cannot rule out the possibility of direct contact with sick poultry.

The health agency said it may never know what happened, partly because witness accounts have not been consistent and the brother's remains were cremated before tests could be performed.

"The situation is always going to have a question mark hanging over it," WHO spokesman Bob Dietz said in Hanoi.

Bird flu has struck poultry in at least 10 Asian countries, but infections in people have been reported only in Thailand and Vietnam.

China announced five new suspected poultry cases Monday, including one in its remote northwestern region of Xinjiang -- underlining the potentially broad range of the disease. Xinjiang is more than 1,000 miles from the southern region where China's first case was confirmed last week.

With the new report, China now has three confirmed cases and eight suspected cases.

WHO has urged China to take swifter action against bird flu, warning that its chances to contain the disease may be dwindling. Beijing has closed poultry markets and processing factories in some affected areas.

Limited person-to-person transmission of the virus is not the real danger. What experts fear is the virus mutating into a form that passes easily between people -- a pandemic strain that is a hybrid of the bird virus and a normal human influenza variety.

Bird flu spread between humans in a 1997 outbreak in Hong Kong that killed six people. Although it passed from infected people to health workers, it soon lost its punch and failed to transmit further.

Symptoms were very mild or nonexistent in those who caught it from patients rather than birds.

Authorities are also battling the disease in Cambodia, Indonesia, Japan, Laos, Pakistan, South Korea, Taiwan. The strain found in Taiwan and Pakistan is different from the influenza hitting the other countries and is not considered a serious threat to humans.

The FAO has appealed for international aid for Asian farmers, particularly in Vietnam, saying they may otherwise resist slaughtering their flocks, a crucial measure in stamping out the disease and preventing its jump to humans.