Sullivan & Cromwell, Shearman & Sterling and Milbank Tweed Hadley & McCloy have been mandated on a $4.4bn (£2.9bn) project finance agreement on between Rio Tinto and the Mongolian government.

The Oyu Tolgoi Underground Mine Development and Financing plan agreed earlier this year between the government of Mongolia, and Rio Tinto subsidiary Turquoise Hill Resources.

Rio's Turquoise Hill Resources arm owns 66% of the Oyu Tolgoi mine, while the Mongolian government owns the remainder. Rio is the operator of the project, located in the Gobi Desert near Mongolia's border with China.

Rio Tinto turned to Sullivan & Cromwell for legal counsel on the financing.

Earlier this year Rio enlisted Australian firm Allens for advice on the plan alongside its own in-house counsel. Rio Tinto first signed an investment agreement with the Mongolian government in 2009.

Allens has advised Rio regularly on the development of the mine over the past five years.

Milbank is advising the lender group on the agreement. The group is comprised of 15 commercial banks – including BNP Paribas, Societe Generale, Standard Chartered Bank and HSBC – alongside international financial institutions and export credit agencies.

Shearman & Sterling represented Erdenes Oyu Tolgoi, the company through which Rio Tinto and the Mongolian government own the mine, on the project financing deal.

The firm's team was led by Hong Kong capital markets partner Matthew Bersani and Hong Kong project finance partner Andrew Ruff alongside project development and finance partner Etienne Gelencser in Tokyo.

Once fully operational the mine is expected to account for approximately 40% of Mongolia's gross domestic product.

The deal is expected to reach financial close in 2016.