France's President Nicolas Sarkozy declared Monday that world powers should toughen their sanctions on Iran following the failure of talks on limiting Tehran's nuclear ambitions.

"Sanctions are beginning to produce their effects. We must reinforce the sanctions," Sarkozy told reporters, while adding that France still had "deep reserves" about Western military action against Iran.

On Saturday, the six-strong contact group of world powers concerned with Tehran's nuclear programme met Iranian representatives in Istanbul, but failed to secure any new concessions from the Islamic republic.

No new talks were scheduled, but Iran has since said it is open to more dialogue, while insisting it will never surrender its right to conduct a civilian nuclear energy programme.

Western powers suspect Iran's bid to refine uranium is aimed at creating weapons-grade fuel for nuclear arms, and Paris and Washington are leading demands for it to halt its enrichment programme.