At least four people have been killed in a helicopter crash at a military base in the US state of Hawaii, according to media reports.

The helicopter took part in an exercise at the US Navy's missile launch and test site near Kekaha on the island of Kauai on Tuesday morning, US broadcaster ABC and other media reported, citing a statement from the military.

The cause of the crash was not initially clear.

The four killed worked for the civilian company Croman, which carries out orders for the US Navy, as quoted by ABC from a company statement.

The helicopter was therefore a Sikorsky S-61N.

Further details were not initially known.

The site in the desert-like west of Kauai is the world's largest instrumented facility for simultaneous ground, air and sea operations, according to the US Navy.