A commercial poultry farm in Delaware with 1.2 million birds has been hit by a highly lethal form of avian influenza, the state said on Wednesday, significantly expanding an outbreak that has killed US chickens and turkeys, reported Reuters.

The outbreak brings the total number of commercial US poultry birds affected by the disease to about 1.65 million over the past two weeks.

Outbreaks hurt producers by killing their flocks and triggering export restrictions. Japan blocked eggs from Delaware in the latest blow to the sector.

All the birds at the infected farm in New Castle County, Delaware, will be culled to stop the spread of the disease and will not enter the food system, state officials said. They added that poultry products are still safe to eat and that humans are not at risk.

The farm was infected with a highly pathogenic H5 type of avian influenza, which causes high mortality in poultry. It is the same strain confirmed recently in commercial poultry operations in Indiana and Kentucky and in wild birds.

Wild birds are thought to have carried the virus to North America. It was already widespread in Europe and affecting poultry in Asia and Africa.

The Delaware outbreak is the state's first case of highly pathogenic avian flu at a commercial poultry farm since 2004, officials said. It is not known exactly how the farm became infected, Delaware Agriculture Secretary Michael Scuse said in a statement.

In Indiana, meanwhile, officials on Wednesday reported a suspected outbreak of highly pathogenic avian flu at a fifth commercial turkey farm. About 36,000 birds at the farm were culled, bringing the total number of affected turkeys in Indiana to nearly 155,000 since 8 February, the state said.