Xstrata says it will declare its $428 million takeover offer for Sphere Minerals unconditional if it attains an interest in the target of more than 50 per cent stake before the bid closes.

The Swiss mining giant said it would allow the $2.50 per share bid to lapse if it failed to gain a controlling stake in Sphere, a Mauritania-focused iron ore mine developer, by November 12.

"We wish to announce our willingness to declare our offer unconditional under certain circumstances in order to expedite the process," Xstrata Coal chief executive Peter Freyberg said in a statement.

Xstrata also said it planned to reduce the time by which it must pay out Sphere shareholders from five business days to three business days after the offer is declared unconditional.

Xstrata's stake in Sphere has actually backtracked, standing at 8.21 per cent today, down from 8.38 per cent previously.

Of this, 5.46 per cent has been contributed by directors of Sphere.