By David Winning

SYDNEY--Rio Tinto PLC said it has reached agreement with Mongolia's government on domestic power supply to the Oyu Tolgoi copper mine.

Rio Tinto said Mongolia will fund and construct a state-owned power plant at Tavan Tolgoi, the location of a large coal deposit.

The revised Power Source Framework Agreement signals construction of the coal-fired power plant should begin before July 1, 2021. Commissioning of the plant is then due to happen within four years, Rio Tinto said.

"This agreement provides a potential pathway to securing a domestic power supply for the Oyu Tolgoi mine and underground project for the benefit of all shareholders and the wider community," said Arnaud Soirat, Copper & Diamonds chief executive.

Oyu Tolgoi currently uses imported power.

Write to David Winning at david.winning@wsj.com