BHP Billiton Plc/Ltd said on Wednesday it was considering possible next moves in its attempt to open takeover talks with rival mining house Rio Tinto, which has so far rebuffed its overtures.

In a statement to the Australian Stock Exchange, BHP also reiterated its belief that its proposal to offer three of its own shares for every Rio Tinto Plc/Ltd share remained compelling and would unlock billions of dollars in synergies.

BHP is considering its possible next steps and continues to talk to shareholders about their views, BHP said.

Rio has rejected BHP's proposal, which it received in the form of a letter, as out of the ballpark, and has declined to meet BHP face-to-face to discuss a merger, valued at around $130 billion based on Thursday's share prices.

BHP was expected to boost its offer by about a fifth, possibly by adding a cash sweetener, a Reuters poll of investors and analysts last month showed.

Rio has rejected it, and I don't think it's going to be successful in its current form, said Fat Prophets mining analyst Gavin Wendt.

Rio on Tuesday said it had asked Britain's Takeover Panel to set a deadline under a put up or shut up rule by which BHP would have to formalize its approach or walk away.

BHP in its statement said it will continue to press for discussions with Rio.

BHP's comments come less than a week after Rio Chief Executive Tom Albanese branded BHP's proposal dead in the water.

Given BHP's recent actions, the starting point is most likely to be a formal 3-for-1 offer, said BT Investment Management analyst Tim Barker.

They must be seriously thinking about bringing out the 3-for-1 if they're being forced, said Barker.

Based on conversations with shareholders, BHP said its proposal needed an assessment of relative values and that a number of shareholders requested more details of the relative performance and outlook for both companies, in particular BHP's contribution to the combination.

If it succeeds in its present form, the merger would be the second-biggest corporate takeover after Vodafone's $203 billion buyout of Mannesmann in 2000.

UBS has said BHP could afford to put a further $27 billion in cash into the offer on top of a promised $30 billion share buyback.

(Additional reporting by Sonali Paul in Melbourne)

(Reporting by James Regan; Editing by James Thornhill)