"Our goal remains a diplomatic solution, one in which Iran's leaders set aside their ambitions for nuclear weapons capabilities," said Gregory Schulte, the U.S. ambassador to the International Atomic Energy Agency in Vienna.

"But inducing the current Iranian leadership to make this choice will clearly require tough and sustained diplomacy, including action by the UN Security Council," Schulte told a forum held by the Geneva Center for Security Policy.

He said a resolution, introduced by Britain and France and backed by Germany and the United States last week, was meant to make the Security Council's requests on Iran mandatory under Chapter VII of the UN Charter.

If Iran failed to heed a Chapter VII resolution, it might face "targeted sanctions," he said, adding that the U.S. and the European Union were already discussing a range of such sanctions.

"For diplomacy to succeed, we must be prepared to use the full range of diplomatic tools available to the Security Council and the international community," the diplomat said.

He urged the European Union as a whole to apply its leverage and called on "other like-minded countries" to work together with the U.S. with unity and resolve in tackling the challenge of Iran's nuclear ambitions.

Schulte also recalled proposals made by Britain, France and Germany last August, which offered Iran assurances of fuel supply for light water power and research reactors in exchange for its abandonment of uranium enrichment activities.

He called on Iran's leadership to reconsider those proposals and choose cooperation and negotiation over confrontation and defiance.