Giant mining firm Rio Tinto Ltd (ASX: RIO) took advantage of a credit facility extended to Ivanhoe Mines Ltd that matured on Tuesday and sued for its conversion into common shares that effectively upped the company's stake in the Canadian firm by almost 35 percent.

In a statement issued on the same day, Rio Tinto said that by converting the credit facility, it has just acquired a total of 40,083,206 common shares of Ivanhoe Mines, which currently owns 63 percent of Australian-based Ivanhoe Australia Ltd, adding up 5.3 percent to its existing shares and leading to 34.9 percent of outstanding common shares.

The giant mining company added that it stopped short of its full purchase warrants that could have led to a total 44 percent interests in Ivanhoe Mines, which acquired the convertible credit facility of $US350 million or $A376.28 million from Rio Tinto in 2007, that according to Rio has given it "the right at any time to exercise its remaining share purchase warrants into common shares of Ivanhoe."

Rio Tinto said that it would continue weighing on the possibility of using its rights to get more Ivanhoe securities but that would mostly depend on its "assessment of Ivanhoe Mines business, prospects and financial condition, the market for Ivanhoe Mines securities, general economic and tax conditions, and other factors."

The already matured credit facility, as agreed by both Rio Tinto and Ivanhoe Mines, was automatically converted into common shares that carry a price of $US10.00 or $A10.70 per security.

Including the outstanding principal and accumulated interest, the credit extended by Rio to Ivanhoe surged to an estimated $US400.8 million at the time of its maturity date.

Both companies are joint explorers of the $US4.6 billion Oyu Tolgoi copper and gold mining project in Mongolia, which was characterised by Rio Tinto as the world's biggest reservoir of untapped copper and gold resources.

Ivanhoe Mines currently maintains 66 percent ownership of the Oyu Tolgoi project, with a further 57 percent stake in a coal mining project, also in Mongolia, and another 50 percent interest in a Kazakhstan gold exploration site.