Iowa reported a highly lethal form of avian influenza in a backyard poultry flock on Wednesday, expanding a US outbreak of the disease to the top egg-producing state, reported Reuters.

The flock in Pottawattamie County, which was not being raised for commercial production, will be culled to prevent the spread of the disease, Iowa said.

Over the past month, highly lethal cases of avian influenza have been confirmed at commercial farms in Indiana, Kentucky and Delaware, triggering export restrictions for US poultry products.

In 2015, Iowa was at the centre of the biggest-ever US outbreak of avian influenza, which killed about 50 million birds.