A White House statement said the two leaders discussed by phone the "good progress" being made by the United States, Russia, Britain, China, France and Germany toward agreeing on a fourth round of UN Security Council sanctions against Iran for refusing to halt its uranium enrichment.

They "agreed to instruct their negotiators to intensify their efforts to reach conclusion as soon as possible," the statement said.

Western nations fear that Iran's nuclear program is aimed at building atomic weapons. Tehran denies the charge, saying it only wants to generate electricity for civilian use.

Russia, like China, has close trade ties with Iran and has been reluctant to support new punitive measures against Tehran. Moscow eventually gave in but is supporting Beijing's push to dilute the proposed sanctions outlined in a U.S. draft.

As negotiators from the six powers work to hammer out a draft resolution, Brazilian President Luiz Inacio Lula da Silva is scheduled to head to Tehran this weekend to help mediate.