A hangar under construction at an airport in the US state of Idaho collapsed on Thursday, killing three people and injuring nine others, Boise city authorities said.

"Three people died on scene, and nine people were injured," Boise city said in a statement, adding five of those injured were in "critical condition".

The accident occurred at a privately owned hangar at Boise Airport's airfield in Idaho's capital.

The city's division chief of operations, Aaron Hummel, described the incident as "a catastrophic collapse of a metal structure at a construction site."

An investigation into the cause of the collapse has been launched, the city said, adding the accident did not affect operations at the Boise Airport.

The site was declared secure and "there is no threat to the public," the city statement said.

The airport is served by multiple airlines that offer flights to 25 destinations in the United States, including San Francisco, Atlanta and Seattle, according to the airport's website.