Still, South Korea's Agriculture and Forestry Ministry said in a statement that it will indefinitely halt customs inspections of U.S. poultry "as a precautionary measure." Last year, South Korea imported 40,107 tons of chicken meat and 87 tons of duck meat from the United States.
Singapore also stopped U.S. poultry imports, the Agri-Food and Veterinary Authority said in a statement. The city-state imported 19,300 tons of frozen chicken meat worth about $19.5 million from the United States last year.
Health officials have said the bird flu is contracted through direct contact with infected birds, but experts have said it's possible the virus jumped to humans through a mammal, like pigs, which have been implicated in past human flu outbreaks.
Swine often are housed with poultry in traditional family farms in Asia, and are more genetically similar to humans than birds are.
Meanwhile, the World Health Organization said the bird flu virus that killed two Vietnamese sisters did not contain human genes, meaning there is still no sign the virus sweeping Asia has mutated into a new, more contagious form.
The women's blood was tested because experts suspected they might have caught the disease from their brother, who also died; but that link could not be proven because the brother's body was cremated. So far, there have been no known cases of person-to-person transmission in the current bird flu outbreak.
Health experts have been most worried about the possibility of the disease combining with the human influenza virus to create a more lethal version that could be spread between people - giving rise to a global pandemic.
The new data is "reassuring" evidence that the H5N1 bird flu virus that's hitting Asia has not acquired that ability, the WHO said in a statement posted on its Web site late Friday.
Vietnamese officials, meanwhile, denied claims that pigs have been infected with the disease that has forced the slaughter of millions of chickens throughout Asia and killed at least 18 people.
China's Agriculture Ministry reported bird flu in three more provinces on Saturday.
The cases were in the provinces of Hubei, Henan and Jiangxi and quarantine measures were imposed, the ministry said in a statement released through the official Xinhua News Agency. Both Hubei and Jiangxi have reported previous cases in fowl in recent days.
The WHO was investigating Cambodia's first suspected human case - a woman who fell ill in Takeo province and died in a hospital in neighboring Vietnam, said Sean Tobin, a WHO medical epidemiologist in the capital, Phnom Penh.
Vietnam's Ministry of Agriculture and Rural Development, which has run separate, extensive tests on pigs in bird flu-affected areas, said their findings have all turned up negative for the H5N1 flu virus, said Bui Quang Anh, director of the ministry's Veterinary Department.
"I can formally announce that no bird flu virus has been found in pigs in Vietnam," he said Saturday.
Anh said 179 samples were taken from pigs in the country's north and south. The samples were then sent to the WHO's Hong Kong laboratories.
He also criticized an announcement Friday by Anton Rychener, Vietnam representative of the U.N.'s Food and Agriculture Organization, who said preliminary tests on nasal swabs taken from pigs in and around Hanoi showed the presence of the H5N1 strain.
"I don't know on what justification FAO made such a statement," Anh said.
Officials at the Rome headquarters of the U.N. agency later downplayed the findings, saying that the results do not necessarily mean the pigs are infected.
The tests may merely be confirming the presence of infected chicken droppings on their snouts. Rigorous tests look for the virus or antibodies in the blood, the agency said.
The avian influenza has killed 13 people in Vietnam, and five in Thailand. More than 50 million chickens have been slaughtered in Asia to stem the spread of the virus.