Authorities in the northeastern US state of Delaware planned to kill all 12,000 birds on a farm which raises chickens to sell at live markets in New York where local media reported on Saturday avian influenza had been found.
Officials suspect the farm's birds were infected in New York, and said all farms within a 3.2 kilometer radius of the farm will be tested for avian flu within the next week, a local newspaper reported.
South Korean ban
Though officials there said the strain was not related to the Asian outbreaks, South Korea announced a temporary ban on the import of poultry and poultry products from the United States.
Kim Chang-Seop, head of the animal quarantine department at the South Korean agriculture ministry, warned the ban "would become an official one" if the strain proved to be highly pathogenic.
China's Ministry of Agriculture late on Saturday confirmed H5N1 bird flu outbreaks were found in Hubei, Henan and Jiangxi provinces, the Xinhua news agency reported.
The ministry also received reports of new suspected bird flu cases in Yunnan and Gansu provinces, bringing the total number of outbreaks of confirmed and suspected bird flu to 31 across China.
Bird flu has so far killed 18 people - five in Thailand and 13 in Vietnam - and caused the deaths or slaughter of millions of birds.
Vietnam on Saturday added its voice to assurances from the UN Food and Agriculture Organisation (FAO) that there was no proof yet that pigs were now infected with bird flu.
On Friday the FAO's Vietnam director Anton Rychener said the highly pathogenic H5N1 virus had been detected in nasal swabs taken from pigs in the capital Hanoi and its surroundings.
Experts say pigs are an ideal "mixing vessel" in which viruses swap genes, become more lethal or contagious and then leap to humans.
The virus can not be transmitted
from humans to humans
But Rychener's comments were swiftly played down by the UN agency's Rome headquarters, which said there was no evidence this was happening in Vietnam.
"At this time we have seen no data that would indicate that pigs are in any way involved in spreading the current strain of H5N1 influenza virus," said Peter Roeder, an FAO veterinary virologist and animal health expert.
Bui Quang Anh, director of Vietnam's agriculture ministry's veterinarian department, said on Saturday blood samples from 179 pigs sent to laboratories in Hong Kong had tested negative for the H5N1 strain of bird flu.
"I can officially declare that we have not found evidence of bird flu in pigs," he told AFP.
At present the virus is transmitted from poultry to humans -
most likely through airborne faeces that carry the bug - but cannot be transmitted from humans to humans.
However, the World Health Organisation has warned that H5N1 could kill millions around the globe if it combined with a human influenza virus to create a new highly contagious strain transmissible among humans.
"At this time we have seen no data that would indicate that pigs are in any way involved in spreading the current strain of H5N1 influenza virus."
Vietnam's latest victim, a six year-old girl, died on Tuesday at a Ho Chi Minh City hospital.
Thailand, the only other country to have confirmed human infections, is also one of those singled out for blame in initially covering up its outbreak before finally coming clean last month.
Thai Prime Minister Thaksin Shinawatra, however, has repeatedly insisted his government had not been remiss in handling the crisis.
He also urged Thais on Saturday to join in a national "Eat Chicken" day aimed at restoring plunging consumer confidence in its poultry sector, but this was overshadowed by the announcement of three more suspected human infections.