Three US firefighters who died when their air tanker crashed while battling blazes in Australia have been identified.

Captain Ian McBeth, 45, first officer Paul Clyde Hudson, 43, and flight engineer Rick A DeMorgan Jr, 44, were killed in New South Wales on Thursday.

The cause of the C-130 Hercules crash is not yet known.

New South Wales (NSW) Premier Gladys Berejiklian called it an "unbelievable loss" and offered a state memorial