The world's largest miner had until today to make a formal bid for Rio after the UK Takeover Panel imposed a February 6 deadline for a formal offer under the "put up or shut up" clause.

Today's formal takeover offer is higher than BHP's initial three-for-one proposal announced in November and values the deal at around $US173.6 billion ($A192 billion).

In a teleconference today, BHP chief executive Marius Kloppers said the bid was BHP's first and only offer for Rio, however he refused to directly answer whether the company would look to increase its bid in the future.

He said the reason behind the increase in its offer to 3.4-for-one was due to a number of factors and not specifically as a result of Chinalco and Alcoa's share swoop on Rio last week.

"We have had approximately three months to gather information," he said.

"I wouldn't pin the decision on that particular element of our offer on any individual action or piece of information. Rather it is the considered response to all the information we have received.

"I think the Chinalco action looms large because it was most recent but so many profound things have happened over the last three months that we have now seemed to have forgotten about.

"Our board and senior management have considered all the developments and have taken account of what shareholders, customers and others have said to us. We have assessed the scenarios and consider the offer in the best interests of all shareholders."

Kloppers said that the company had the finance in place for the takeover deal.

"We have the means in place to complete this transaction and we have that in place on a very conservative baseline assessment," he said.

According to BHP, the offer represents a 45% premium to the Rio share price prior to BHP's announcement of its takeover proposal last year.

"The way we look at it, we were proactive in unlocking value for the Rio Tinto shareholders," Kloppers said.

"Those same shareholders will now ask of the Rio Tinto board why they are not allowing them to realise this substantial uplift."

Kloppers said he would not comment on whether he had met with Chinalco representatives.

Meantime, Rio Tinto's chairman Paul Skinner said the company would consider the terms of the proposal carefully and will make a further statement once they have completed this assessment.

"In the meantime, the boards encourage shareholders not to take any action," he said.

The deal to is expected to create synergies and benefits which will contribute earnings before interest, tax, depreciation and amortisation (EBITDA) of $US3.7 billion within seven years of the deal going through.

BHP's first approached Rio on November 1 last year and since then Rio has declined to negotiate with BHP, saying the proposal undervalued the company.

Shares in BHP remain unchanged at $39.65, while Rio shares are trading at $127.35.