WASHINGTON — Iran and six world powers have agreed on a plan to launch the first phase of their nuclear deal, U.S. and Iranian officials announced Sunday, a sign the fragile international effort to curb Tehran's nuclear ambitions remains on track.
The agreement, worked out in a month of talks between technical experts and diplomats, goes into effect Jan. 20, after which Iran will begin to get access to some of its frozen assets -- and will have to open more of its facilities to the United Nations' nuclear watchdog agency.
Meanwhile, Iran and six world powers led by the United States will try to negotiate a long-term agreement to limit Iran's nuclear development.
Diplomats had been working toward the Jan. 20 date for some weeks. The date also starts a six-month period in which the seven countries will try to negotiate the longer-term deal.
Officials of the United States, Iran and the European Union hailed the agreement as an important move.
The agreement marks "concrete progress," said President Obama. "I welcome this step forward, and we will now focus on the critical work of pursuing a comprehensive resolution that addresses our concerns over Iran's nuclear program."
Success in reaching the implementation deal could help restrain hawks in Washington and Tehran who have argued that the Nov. 24 agreement threatens their interests, analysts said.
However, it was not immediately clear whether the terms set for Iran were strict or lenient.
Iran and the six powers signed a preliminary agreement Nov. 24 that was vaguely worded and left much to be decided later. The text of the implementation agreement was not released.
The initial agreement calls for Iran to halt enrichment of medium-grade uranium -- which can be easily refined to bomb-grade. It also requires a halt in the installation of additional centrifuges and temporary neutralization of Iran's stockpile of medium-enriched uranium.
The agreement calls for Iran to halt key work on a half-built heavy-water nuclear reactor at Arak, which the West finds especially threatening. But the deal allows Iran to continue with some work at Arak deemed secondary.
The new implementation plan gives instructions to the International Atomic Energy Agency, or IAEA, on how it is to conduct inspections of Iran's nuclear sites to verify that Tehran is complying with the terms of the preliminary deal. The deal calls on Iran to allow increased inspections of its uranium enrichment sites, its centrifuge manufacturing facilities and the Arak heavy-water facility.
A key point of contention during recent talks has been whether Iran could install more advanced centrifuges in its enrichment plants. Obama said in his statement that Iran had agreed not to "use next-generation centrifuges."
The agreement does, however, allow Iran to continue current research and development work on the centrifuges, including some advanced models, a senior administration official said.
Abbas Araqchi, Iran's chief negotiator, said that the two sides had agreed to allow Iran to continue using one kind of advanced centrifuge at its Natanz enrichment plant, Iranian media reported.
U.S. officials, who asked not to be identified in discussing details of the undisclosed document, said Iran will be allowed to tap frozen overseas funds incrementally over the next six months, as monthly IAEA reports show that it is complying with curbs on its nuclear program. In all, Iran is to receive about $7 billion in relief over the period.
Failure by Iran to comply with the implementation plan would not necessarily void the Nov. 24 agreement, officials said. But it would halt sanctions relief and could cause the United States to impose additional sanctions, they said.
Completion of the deal comes at a tense moment in Washington, where 59 senators have signed on to a new Iran sanctions bill that the White House says could destroy the negotiations.
In his comments, Obama warned that he would veto any sanctions legislation, saying that "imposing additional sanctions now will only risk derailing our efforts to resolve this issue peacefully."
On Thursday, the White House, in its toughest language yet, suggested that sanctions advocates had a hidden agenda of starting a war with Iran.
One of the chief sanctions advocates, Sen. Mark Steven Kirk (R-Ill.), sought to turn the tables Sunday, arguing that the administration's approach could lead to war.
"I am worried the administration's policies will either lead to Iranian nuclear weapons or Israeli airstrikes," Kirk said in a statement. "Beginning Jan. 20, the administration will give the world's leading state sponsor of terrorism billions of dollars while allowing the mullahs to keep their illicit nuclear infrastructure in place."
Kirk also urged the administration to release the implementation agreement.
Araqchi, the Iranian negotiator, said that if Congress adopted new sanctions, "the world will know who was responsible for the failure" of the negotiations.
Special correspondent Ramin Mostaghim in Tehran contributed to this report.