Some of the world's biggest mining companies are readying for a fight over proposed changes to mining laws in Indonesia that threaten to undermine operating agreements and boost local ownership of mines at the expense of foreign companies.

Rio Tinto, Freeport McMoran and Australia's Newcrest Mining have all confirmed that the proposed changes may affect their Indonesian assets, with Rio chief executive Jean-Sebastien Jacques telling investors on Wednesday that it was a "serious" and "challenging" situation.