Bangkok, Thailand ? Two Vietnamese sisters who died from bird flu may have caught the disease from their brother, which would be the first known case involving human-to-human transmission in the outbreak now sweeping Asia, the World Health Organization said Sunday.

The source of the sisters’ infection has not been identified, but investigations have failed to find a specific event, such as contact with sick poultry, or an environmental source to explain the cases, WHO spokesman Bob Dietz said in Hanoi.

“Limited human-to-human transmission from the brother to his sisters is one possible explanation,” he said.

No other cases of people catching the virus from other people have been suspected anywhere else.

Bird flu has killed millions of chickens in 10 Asian countries and jumped to humans in Thailand and Vietnam, killing at least 10 people.

China closed poultry markets and processing factories in bird flu-affected areas shortly after WHO warned that Beijing’s chances to contain the disease may be dwindling.

WHO called on China to share more information about the disease, step up monitoring for possible human cases and take precautions so workers slaughtering birds are not infected.

The U.N. Food and Agriculture Organization appealed for international aid for Asian farmers, saying they may otherwise resist slaughtering their flocks, a crucial measure in stamping out the disease and preventing a human outbreak.

“We are … concerned that mass culling is not taking place at a speed we consider absolutely necessary to contain the virus,” said Hans Wagner, an FAO animal production and health officer.

Limited human-to-human 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.

There is no evidence that a new strain has emerged, WHO spokeswoman Maria Cheng said. Results from tests comparing the genetic makeup of the virus found in the two sisters with that found in other people are expected from Hong Kong in several days, she said.

“This may be an isolated incident. These were very close contacts, family members,” she said. “We wouldn’t be surprised if we saw more of these cases, especially where you cannot trace the contacts back to chickens.”