CHICAGO (REUTERS) - A US outbreak of a deadly strain of bird flu accelerated on Wednesday with the infection of a sixth turkey flock in Minnesota, the nation's top turkey-producing state, in less than a week.

The infected flock of 310,000 turkeys was the ninth case of the H5N2 flu in Minnesota in just over a month and the biggest flock yet to be confirmed with the disease, according to the US Department of Agriculture. The virus was detected at a facility west of Minneapolis that is owned by a subsidiary of Spam maker Hormel Foods Corp, the company said.

Since the beginning of the year, the flu, which can kill nearly an entire poultry flock within 48 hours, has also been found in birds from Oregon to Arkansas. The discoveries have prompted major overseas buyers such as Mexico and Canada to limit imports of US poultry and companies such as Tyson Foods Inc to strengthen measures to keep the disease off farms.

The number of infections is climbing as migratory ducks, which are believed to be spreading the disease, return to Minnesota to breed after spending the winter farther south, said Beth Thompson, assistant director of the Minnesota Board of Animal Health. The larger number of ducks likely increases the risk for wild birds to transmit the virus.

Farm workers are probably infecting turkeys by tracking the virus into barns after stepping in contaminated duck feces, said John Glisson, vice president of research for the US Poultry and Egg Association. Chicken flocks are also vulnerable. "Minnesota is a real hotbed for returning waterfowl,"Glisson said.

The USDA has said it believes migratory ducks are spreading the flu and sent a team to Minnesota to determine how it is moving into poultry flocks.

So far, efforts to stop the spread by controlling human and vehicle traffic on farms have not worked.

The number of infections may continue to rise through mid-May, when spring migration ends, said Steve Olson, executive director of the Minnesota Turkey Growers Association. New cases may accelerate again in the autumn when recently hatched ducks, which have never been exposed to the virus, begin migrating south, he added.

The H5N2 flu will likely remain a threat to US poultry for three to five years, Olson said, citing information from wildlife experts. That is how long it will take wild birds to develop immunity to the disease.

No human cases have been detected.