He said BHP had not spoken with the Rio board before making the offer, but believed the offer had widespread support among Rio shareholders, 60 to 70 percent of whom also hold BHP shares.
The offer equates to a 45 percent premium to Rio's stock price in November before BHP first raised the idea of a union in a three-for-one share swap.
"It's a lot fairer than the offer we've had before. It's by no means a knock-out offer," said Bertie Thomson, a fund manager at Aberdeen Asset Management, who holds both Rio and BHP shares.
Shares in BHP fell 4.8 percent at A$37.75 on Wednesday, while Rio fell 0.3 percent to A$128.96.
Chinalco's Next Move
Chinalco and Alcoa, which last week built a surprise 9 percent stake in Rio Tinto, said they were watching for further developments after BHP made a bid for Rio.
"Together, we plan to closely monitor further developments, in particular any response from the board of Rio Tinto. As shareholders in Rio Tinto plc, we believe any offer should reflect the fundamental value of the company," the firms said in a statement by their Singapore-based vehicle Shining Prospect.
The statement made no mention of any possible counter bid, but London newspaper the Times, citing unnamed sources close to Chinalco, said on Wednesday a deal was in the works.
The newspaper said Chinalco was seeking approval in Australia to raise its holding to 19.9 percent and may go further to ensure the BHP bid is blocked.
However, sources familiar with the situation told Reuters that Chinalco is not currently mounting a bid for Rio, adding that the Chinese firm is in no rush to make the next move.
Rio Tinto has long opposed BHP's overtures, arguing it was better off as an independent company, digging its own iron ore mines and churning out hundreds of thousands of tons of copper, zinc and aluminum.
Rio in a statement said it was considering the offer and advised its shareholders not to take any action.