An 18-year-old man died in Vietnam, making it the worst affected country with nine deaths, while a six-year-old boy and a 58-year-old woman became the latest victims of avian influenza in Thailand.
The WHO had said earlier the H5N1 virus could combine with the human influenza virus to create a new entity that would be hard to contain and could kill millions of people.
Ten Asian nations have reported bird flu cases since December and the virus has led to the deaths of 12 people, nine in Vietnam and three in Thailand.
The WHO said two Vietnamese sisters who died on 23 January in Hanoi could have contracted the lethal H5N1 strain of bird flu from their dead brother.
"The investigation has not been able to conclusively identify the source of infection for the two sisters," WHO said in a statement.
"WHO considers that limited human-to-human transmission, from the brother to his sisters, is one possible explanation."
However, the health body added there was "no evidence of efficient human-to-human transmission of H5N1 occurring in Vietnam or elsewhere".
Thai PM criticises WHO
Thai Prime Minister Thaksin Shinawatra on Monday criticised WHO for suggesting that human-to-human transmission took place in Vietnam.
Thaksin (R) has urged scientists
not to make wild claims
He said researchers should go public with their theories only if there was a good chance of being proved right.
"Normally the ethics of researchers is such that if there is only a slight possibility of something happening, then they will discuss it among themselves, they will not say anything to the public to raise concern," he told reporters.
"If the possibility is higher than 5% they should say something, but if it's under 5% they should not say anything. The possibility of human-to-human transmission is 0.00001%."
Three more deaths
A six-year-old Thai boy suspected of having bird flu died in hospital on Monday, shortly after the death of a 58-year-old woman, taking the country's toll from the disease to three, officials said.
Thailand now has four confirmed cases of bird flu, including three people who have died, and another 18 suspected cases, of whom 10 have died, said disease control department director Charal Trinvuthipong.
"Another suspected case died at noon today at the Queen Sirikit National Institute of Child Health," Charal said, adding that the boy was from western Kanchanaburi province, which is one of the worst-hit regions.
In Vietnam, an 18-year-old man died in a Ho Chi Minh City hospital early on Monday, Tran Tinh Hien, deputy director of the city's Tropical Disease Hospital, said.
"He was admitted to our hospital on Thursday and on Saturday he tested positive for H5N1," the official said.
"According to our information he had direct contact with chickens in his village."
China on guard
In China, authorities have reported 14 suspected or confirmed cases and have stepped up efforts to contain the disease.
Chinese authorities have stepped
up surveillance against bird flu
Officials ordered the killing of chickens, closed down poultry markets and stepped up surveillance, as the WHO urged speedy measures to curb the virus.
"It's entirely conceivable that there could be more cases among poultry populations," WHO spokesman Roy Wadia said.
"It seems to be spreading very fast. Time is of the essence, and you have to keep up with the outbreak," he added.
In Shanghai 2000 people have been quarantined and 35,000 ducks culled after 200 of the birds died at a farm outside the city.
Compensation for farmers
The Chinese government has also begun compensating farmers whose poultry has been culled.
In central Hubei province, 3600 farmers will receive money, and the authorities have raised over $122,000 in compensation.
District governments in Shanghai are expected to spend $723,000 to compensate farmers whose 300,000 birds were culled.
Cambodia, China, Indonesia, Japan, Laos, South Korea, Thailand and Vietnam have all reported outbreaks of H5N1, while Taiwan and Pakistan have reported weaker strains.