Xstrata is tightening its grip on Sphere Minerals after major shareholder Genesis Investment Management agreed to sell its 7.98 per cent stake in the company into the Swiss miner's $3 offer.

The decision means Xstrata now holds more than 46.5 per cent of Sphere.

Not to be outdone, rival bidder Sin-Tang Development announced today it currently held 13.5 per cent of the company and had received Foreign Investment Review Board approval to increase its stake to 19.9 per cent and a further 3 per cent subject to creep provisions of the Corporations Act.

Sphere has advised its shareholders to sell into Xstrata's sweetened $3 cash offer.

It has described Sin-Tang's rival $460 million financing plan as conditional and inferior to Xstrata's cash offer.

Sphere has three large-scale iron ore projects in Mauritania in north-western Africa.

Sin-Tang has offered to secure funding for Sphere to develop its Askaf iron ore project in Mauritania, one of Sphere's three large-scale iron ore projects in north-western Africa.

The Sin-Tang proposal is not a takeover bid, but involves an equity raising seeking $110 million at $3.25 per share.

Xstrata has been pushing to secure a 50 per cent stake in Sphere before a deadline on Friday, at which time its bid will expire if it has not received sufficient acceptances.