Six people have been killed in a mid-air collision involving two medical helicopters.

The Bell 407 helicopters collided in northern Arizona about half-a-mile from the Flagstaff Medical Centre.

One of the aircraft - operated by a company called Air Methods from Colorado - was on its way to the centre with a patient who died in the crash along with two others.

A flight nurse onboard survived the crash but is in a critical condition.

The second aircraft - owned by Classic Helicopters in Utah - was carrying a patient with a medical emergency from the Grand Canyon's South Rim. All onboard were killed in the collision.

Emergency services carried the injured from the wreckage on stretchers.

Medical centre staff said it was highly unusual to have two helicopters landing at the site.

The facility does not have flight controllers and pilots are suppose to navigate their own approach.

It is not yet know what caused the accident but Classic Helicopters said the pilot of its aircraft was very experienced with over 10,000 hours of flight time.

A federal investigation report is not expected until next year.

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