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. "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 was investigating whether two Vietnamese sisters who died from the disease last month may have caught the disease through contact with their brother. If so, that would be the first human-to-human transmission in this outbreak.
WHO said there was no evidence yet of a new strain that can easily be passed among people.
Investigators have failed to trace the sisters' infection to a specific event, such as contact with sick poultry, or an environmental source. Neither human-to-human transmission nor direct contact with sick poultry can be ruled out, WHO says.
Health officials may never be able to confirm what happened, partly because the brother's remains already have been cremated.
"The situation is always going to have a question mark hanging over it," WHO spokesman Bob Dietz said in Hanoi, Vietnam.
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 human-to-human transmission of the virus is not the real danger. Instead, experts fear the virus could mutate into a form that passes easily between people.
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.
WHO was encouraged by tests showing that the bird flu has been in Asia since at least April without causing a large-scale human infection.
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.