Rio Tinto and its partners are exploring ways to remove a $US6 billion cap on debt attached to the Oyu Tolgoi copper project, amid fears the project could cost $US1.9 billion ($2.7 billion) more than planned.

Preliminary talks have been held with some of the lenders behind an existing $US4.4 billion finance package for the mine expansion, which Rio estimated would cost $US5.3 billion to build when it made a final investment decision in 2016.