Rio Tinto's flagship iron ore division has confirmed an 8 per cent slump in exports, while the miner conceded its Mongolian copper project could suffer cost blowouts of up to $US1.9 billion ($2.69 billion).

Rio had expected an underground expansion of Mongolia’s Oyu Tolgoi mine to cost $US5.3 billion when it was approved in 2015, but has known for the best part of a year that the project would be delayed.