Rio Tinto has competition for Riversdale

Rio Tinto's bid for Riversdale Mining has some competition, with majority shareholder, Tata Steel mulling an offer.

The Economic Times is reporting Tata may explore a joint bid with state-owned mining company NMDC, but both are waiting for Rio to launch a formal bid.

But NMDC's Chairman denies this is the case.

It comes after Riversdale confirmed it's participating in takeover talks with Rio - with hints the mining giant may offer 15 dollars a share.

Tata Steel owns a 24 per cent stake in Riversdale.