CHICAGO (Reuters) - Iowa Governor Terry Branstad declared a state of emergency on Friday due to a rapidly expanding avian flu outbreak, saying the entire state was at risk from the spread of the disease.

The announcement, which gives authorities powers to enforce preventative measures, was made soon after state agriculture officials announced four new poultry farms had initially tested positive for the virus.

Iowa, the top egg-producing state in the United States, is the third state to declare a state of emergency because of the viral outbreak, which either has led or will lead to the extermination of up to 21 million chickens and turkeys nationwide.

Minnesota and Wisconsin declared states of emergency in April.

Iowa's state of emergency is effective immediately now and will be in force until at least the end of May, depending on developments in the outbreak, Branstad told a news conference.

The measure expands the efforts of the state's emergency response plan, and authorizes various state entities access to additional resources, supplies and equipment to track and contain the influenza outbreak. It also allows for the removal and disposal of infected animals on either public or private lands and lifts weight restrictions on trucks hauling culled flocks, among other things.

In addition, the action allows the state and local law enforcement to set up checkpoints and road blocks anywhere in the state, including areas outside of quarantined farms.

"While the avian influenza outbreak does not pose a risk to humans, we are taking the matter very seriously," Branstad said in a statement.

Dozens of countries have imposed total or partial bans on U.S. poultry and poultry imports since the outbreak of avian influenza was discovered in December.

As of Friday afternoon, Iowa officials said, 21 farm sites in 10 Iowa counties had been identified as having either confirmed or presumed positive cases of the highly pathogenic H5 strain of bird flu.

The tally includes the nine Iowa farms that tested positive in the past 24 hours, including a commercial egg operation housing up to 5.5 million birds in Buena Vista County, and a separate egg-laying farm that houses 1 million birds in Madison County, according to Iowa's agriculture department.

So far, an estimated 16 million egg-laying chickens in Iowa are in infected or presumed infected farm facilities - meaning that at least one-quarter of the state's flock will have to be killed and disposed of, state officials said.

(Reporting By P.J. Huffstutter; Editing by Frances Kerry)