Mining group Rio Tinto says a takeover approach of about $130bn (£63.2bn) from BHP Billiton is "a couple of ballparks away" from its true value.
Rio Tinto is to spend on new mines as it seeks to fend off BHP
Rio chief executive Tom Albanese said BHP's all-share move did not recognise his company's future prospects.
He told Australia's ABC TV station that the market agreed Rio was worth more.
BHP Billiton has offered three of its shares for every Rio Tinto share, and Rio shares are trading at about a 17% premium to the offer.
"We've seen the market move well past the BHP proposal, which we rejected, and the market to some extent is speaking," Mr Albanese said.
"It's basically saying that: 'Yes, we agree it's worth a lot more,'" he said.
He did not offer an opinion of Rio Tinto's worth, but said it was "ballparks away" from the BHP proposal.
If the approach were to succeed, it would be the world's second-biggest takeover deal after mobile phone giant Vodafone's purchase of Mannesmann in 2000.
Rio has said it will spend at least $2.4bn on new mines and increase its dividend as it battles to fend off the bid from BHP Billiton, the world's biggest mining group.
Analysts say that Rio's refusal to team up with BHP is indicative of its conviction that mineral and metal prices will stay high for years to come, fuelled by strong demand from emerging economies such as China and India.