Global mining giant Rio Tinto Ltd has decided to extend its $3.9 billion takeover bid for Africa-focused collier Riversdale Mining Ltd following the latest move by the target's shareholder of upping its stake on the coal miner.

Giant Brazilian steelmaker CSN surprised the market on Thursday when it increased its holdings on Riversdale Mining to 19.9 percent, barely a shade from the level would initiate a compulsory takeover proposal.

The seeming emergence of a possible rival to its acquisition plan for Riversdale prompted Rio Tinto to extend its offer through March 4 even as investors of the mining behemoth has indicated that CSN appears far from posing a serious threat on the company's intent on Riversdale.

Rio Tinto's firm movement in the sharemarkets, according to analysts, points to concerns that the miner may not be able to muster the minimum share requirement of 50.1 percent from Riversdale due to the puzzling move by CSN.

However, sources from Rio Tinto has allowed that Riversdale has given its assurance that no offer has yet to emerge despite the latest indications being played out from the CSN camp, which experts said could be undertaking some posturing in order to attain a better deal from Rio Tinto in securing its required coal supplies.

Next to India's Tata Group, CSN holds the second largest share on Riversdale and according to industry experts is on the lookout for acquisition targets with a deep budget of $7 billion.

Also, analysts said that the CSN move could be meant for Rio Tinto to increase its proposal price for Riversdale basing on the earlier pronouncements of the former that its major objective in placing a stake on the coal miner is to zero in on coking coal supplies for its steel mills.

Reports had indicated too that Rio Tinto, CSN and Tata Steel have had their share of talks regarding the deal but only Tata seemed committal so far of supporting Rio Tinto's bid on Riversdale, while CSN has been silent on giving out anything solid.