While Russia remains committed to Bushehr, U.S. officials say work has slowed and Moscow still has not delivered critical nuclear fuel for the reactor.

Robert Einhorn, a former top U.S. nonproliferation official, said a nuclear agreement with Russia is a sound idea "but whether by itself it will constitute a sufficient carrot for Russia to take a tougher position on Iran remains to be seen."

The United States has as much to gain from an agreement as Russia, and Moscow is unlikely to make major concessions just to get it, "but it does provide an incentive for Russia to want to benefit from cooperation with the U.S.," said Einhorn, of the Center for Strategic and International Studies.

A recent bi-partisan study by the Council on Foreign Relations concluded that Russia is "the only power that can effectively threaten Iran with nuclear isolation if it continues to build sensitive nuclear fuel-cycle facilities."

It urged Washington to negotiate a nuclear energy agreement, saying while "it should not be necessary to buy Russia's support, successful cooperation does have to rest on mutual confidence" and this can be strengthened by a stronger legal framework for cooperation on nuclear issues.

The nuclear deal will allow expanded cooperation, including the administration's Global Nuclear Energy Partnership Initiative, and "will reflect Russia's status as a major factor in nuclear commerce, from fuel supply and storage to reactor sales and advanced research," the study said.

Since the end of the Cold War, the United States and Russia have worked to reduce nuclear weapons risks, including a highly successful program under which Russia is blending down 500 tons of weapons-grade material so it can be used to generate electricity rather than for weapons.

The United States, backed by Britain and France, favors limited sanctions if Iran refuses to halt enrichment very soon. Russia and China, the Security Council's other two veto-holding permanent members, have so far opposed this.