Mongolia's Oyu Tolgoi project with Rio Tinto PLC and Ivanhoe Mines Ltd. could end up in the pits after both miners flatly rejected the government's request to reopen the deal to boost Mongolia's stake from 34 percent to 50 percent.

In a statement released Tuesday, Rio and Ivanhoe told the Mongolian government they "are not prepared to renegotiate the investment agreement." Both miners received late last week a letter from the Mongolian Cabinet asking to review "potential changes to the Oyu Tolgoi investment agreement."

Rio and Ivanhoe wrote to Mongolia's National Security Council -- the president, the speaker of the Parliament and the prime minister -- asking them to uphold the deal.

The investment agreement, which was signed in October 2009, outlines a provision allowing Mongolia to hike its claim to the figure it wanted. But this only permissible after 30 years from the agreement's effective date and must be agreed to by all three parties involved - Rio, Ivanhoe and the Mongolian government.

Further, the miners pointed out that the investment agreement took time to be approved and signed since it had to be presented to the Mongolian Parliament several times for review and comment to ensure a fair allocation of value from the project to all stakeholders.

In their response letters, Rio and Ivanhoe warned that should Mongolia continue to push for the unscheduled amendments, it could imperil not only the Oyu Tolgoi project but Mongolia's relations with future foreign investors.

The Oyu Tolgoi investment project is 66 percent owned by Ivanhoe Mines Ltd. and is only halfway through completion. Ivanhoe is 49 percent owned by Rio Tinto.

The Oyu Tolgoi copper-gold mine, touted to become one of the world's five biggest copper mines, is expected to produce an average of 450,000 tonnes of copper and 330,000 ounces of gold per year over the next 10 years. It is projected to account for one-third of total Mongolian GDP by 2020.

Ivanhoe Mines Ltd. spent more than six years negotiating with Mongolia before an agreement was signed. The site is expected to open in 2013.