Anglo-Australian mining giant Rio Tinto on Thursday improved its bid price for Riversdale Mining Ltd. and extended the offer period in what it said would be its final offer.

Rio's boosted its bid to Aus$16.50 from Aus$16 after sluggish take-up of its US$3.9 billion offer for the Mozambique-focused steel-making coal miner by shareholders.

But Rio said the increased offer would be conditional on obtaining interest from more than 50 percent of Riversdale's shareholders by March 23, and that it would not boost its offer beyond that point without a competing proposal.

"The increase in the offer price... gives Riversdale shareholders a highly attractive premium and now is the time for them to accept our recommended bid," Rio Tinto Chief Executive Doug Richie said.

"There has been no sign of a competing proposal in the 11 weeks since the bid was announced. The choice for Riversdale is clear -- accept the ... offer or risk seeing their share price return to pre-bid levels."

The latest offer is 17 percent above Riversdale's closing price of Aus$14.10 before it confirmed it was in talks with Rio. Rio Tinto said Thursday it had extended the offer period by two weeks to April 1.

Australian authorities in January approved Rio Tinto's bid for Riversdale, clearing the main regulatory obstacle for the deal to go ahead, leaving shareholder approval as the last major hurdle.

Rio Tinto bid in December for Australian firm Riversdale's huge coking coal deposits in the southern African nation.

Industry analysts have said the African country will be the second-largest producer of coking coal within the next 15 years, behind Australia.

A consortium of five Indian state-run companies -- International Coal Ventures -- has previously shown interest in Riversdale and is mulling an offer, India's Economic Times reported in December.

Brazilian giant Vale, which already has significant interests neighbouring Riversdale's Mozambique tenements, has also been named as a possible suitor.

The bid is Anglo-Australian giant Rio's first attempted acquisition since the collapse of a huge iron ore tie-up with rival BHP Billiton in October.

Its last big buy, the 2007 takeover of Canada's Alcan, was expensive and hugely unpopular.