Rio already owns 51% of Turquoise Hill, the owner of 66% of the Oyu Tolgoi copper-gold mine in Mongolia.

The company made a $2.7 billion, or C$34 per share, takeover proposal for the shares in Turquoise Hill it doesn't already own in March.

Turquoise Hill has appointed a special committee to consider the proposal.

The funds will be available to Turquoise Hill at any time between now and December 31 and will allow the company to meet its funding commitments for the US$7 billion Oyu Tolgoi underground expansion.

Last week Turquoise Hill said it had available liquidity of $600 million, enough to meet funding requirements until October.

Rio has agreed to extend the date for Turquoise Hill to complete an equity raising of at least $650 million from the end of August to the end of December.

The agreement does not prohibit Turquoise Hill from raising equity though Rio has advised it would withdraw its proposal if the company went to the market for cash.

Speaking at the BofA Global Metals, Mining & Steel Conference in Miami this week, Rio CEO Jakob Stausholm said the ownership structure of Oyu Tolgoi was complex.

"We believe we have made a very attractive offer to [Turquoise Hill] shareholders but we're also very respectful of the very clearly laid-out process in Canada so we'll have to await that process," he said.

Stausholm said Rio would remain disciplined.

"We thought we should give the shareholders a choice," he said.

"We believe it's the right thing to simplify the structure."

Turquoise Hill shares traded at more than C$38 after the offer was announced. The stock dropped 4.9% overnight to $34.16.