Fortescue Metals Group's full-year profit has crashed almost 90 per cent despite the miner's rapid race to cut costs to stay well ahead of the spiralling iron ore price.

Underlying earnings more than halved to $US2.5 billion ($3.43 billion) in the year ended June 30 from the year-earlier $US5.6 million. Full-year profit cratered to $US316 million, which was down 88 per cent from the $US2.74 billion posted for financial 2014.