British Prime Minister Gordon Brown is warning Iran it will face tougher sanctions if it continues to defy the international community over its nuclear programme.
Mr Brown told a nuclear energy conference in London on Tuesday that Tehran remains a serious threat.
He also urged "clear, tough and immediate" sanctions for those who break rules designed to stop the spread of nuclear weapons.
But he also recommended a massive expansion of civil nuclear power worldwide.
Mr Brown said the world must find ways to help non-nuclear states obtain civilian nuclear power to meet energy needs.
He said a new international system has to be found "to help non-nuclear states acquire the new sources of energy they need."