International inspectors say Iran is making good on its promise to stop enriching uranium to the point where it could be used to make nuclear weapons.
In a statement, the White House called the development an important step forward.
In November, Iran agreed to stop enrichment of uranium to 20 percent purity, and to dilute its existing stockpiles of weapons grade uranium.
In return, the United States and its European allies will suspend almost 7-billion of the $100-billion dollars in economic sanctions that have seriously hurt Iran's economy.
Iran has denied its nuclear program is for military purposes.
United Nations inspectors will make daily visits to key nuclear facilities for the next six months, in order to make sure Iran continues to fulfill its pledge.
In another development, the Secretary General of the United Nations invited a team of Iranian delegates to attend a peace conference on Syria this week.
But the U.S. says Iran should not attend the talks unless it meets certain conditions, and Syrian opposition groups are threatening to boycott the meeting unless Iran’s invitation is withdrawn.