The US says Iran has taken a "significant step" towards meeting its nuclear commitments, by sending a large quantity of uranium abroad.
Secretary of State John Kerry spoke after Iran dispatched a shipment of more than 11 tonnes of low-enriched uranium to Russia.
Iran signed a deal in July with six international powers.
It is designed to demonstrate that Iran is not on the threshold of being able to make an atomic bomb.
Under the deal, Iran agreed to reduce its holdings of low-enriched uranium; sharply reduce its quantity of centrifuges - used to enrich uranium - and cut its output of plutonium by re-engineering a power plant.
Mr Kerry said Monday's shipment of low-enriched uranium to Russia had tripled the amount of time it would take to produce enough fuel for a bomb - from two or three months to nine months.
"I am pleased to report that we have seen important indications of significant progress towards Iran completing its key nuclear commitments under the deal," Mr Kerry said.
Under July's agreement, the International Atomic Energy Agency will decide when Tehran has complied with its obligations.
After this, the US and other world powers will begin to dismantle their programme of economic sanctions against Iran.
Key areas of the nuclear deal:
Uranium enrichment: Iran can operate 5,060 first generation centrifuges, configured to enrich uranium to 3.67%, a level well below that needed to make an atomic weapon. It can also operate up to 1,000 centrifuges at its mountain facility at Fordow - but these cannot be used to enrich uranium.
Plutonium production: Iran has agreed to reconfigure its heavy water reactor at Arak, so that it will only produce a tiny amount of plutonium as a by-product of power generation, and will not build any more heavy water reactors for 15 years.
Inspections: International monitors will be able to carry out a comprehensive programme of inspection of Iran's nuclear facilities.
Sanctions: All EU and US energy, economic and financial sanctions, and most UN sanctions, will be lifted on the day Iran shows it has complied with the main parts of the deal.