BHP Billiton has raised its offer for rival Rio Tinto to US$147.4bn as it pushes to create a mining super giant.
BHP's move meets a deadline set by the UK Takeover Panel to make a formal offer today or walk away for six months.
Rio Tinto says it is considering the offer and has not given a timescale for its final decision.
"The boards of Rio Tinto will consider the terms of the proposal carefully in the light of all circumstances," says Rio Tinto's Chairman Paul Skinner.
"In the meantime, the boards encourage shareholders not to take any action."
The proposed deal has raised concerns in China that BHP could gain too much power setting prices, writes Australia's AFP.
Last week, state-owned aluminium giant Chinalco announced a 12 percent stake in Rio together with Aloca, throwing BHP's takeover plans into chaos.
Although the company confirmed it does not plan to make an offer for Rio Tinto, it reserves the right to do so within the next six months.
BHP has placed its unsolicited bid to acquire all of Rio's share capital at a ratio of 3.4 BHP Billiton shares per Rio Tinto share, consisting of 80 percent BHP Billiton plc shares and 20 percent BHP Billiton Limited shares.
By Ozge Ibrahim
