Rio Tinto has offered $C578 million ($A602.75 million) for Canadian junior uranium explorer Hathor Exploration in an all cash deal.

Rio Tinto and Hathor have signed a support agreement for the offer, which would see Rio pay $C4.15 ($A4.33) for each Hathor share.

Rio Tinto’s offer trumps a previous bid for Hathor by Canada and United States-focused uranium miner Cameco.

“Rio Tinto’s strategy is to invest in the primary uranium producing regions of the world to develop long-life, low-cost operations,” the company said in a statement.

“The acquisition of Hathor provides a quality opportunity to expand the Rio Tinto presence in the Athabasca Basin which currently provides approximately 20 per cent of global uranium production.”

Under the support agreement, Hathor can terminate the Rio Tinto deal in favour of an unsolicited, superior offer from another party.

Rio Tinto’s offer is open for no longer than 35 days.