WASHINGTON — Iran and six world powers took a significant step toward nuclear rapprochement on Sunday, announcing a deal to implement a landmark agreement that caps Iran's disputed nuclear program in return for a modest easing of crippling economic sanctions.
The announcement follows up on the breakthrough agreement reached in Geneva late last year after a decade of rising animosity and suspicion between Iran and much of the rest of the world over the country's advanced nuclear development. The six-month agreement halts the most worrisome nuclear work and rolls back some of Iran's sophisticated advances, but it stops far short of ensuring that the country can never develop a weapon if it chooses to do so.
During the six-month agreement, which takes effect Jan. 20, the six world powers — the United States, Britain, China, France, Germany and Russia — will continue negotiations with Iran on a permanent deal.
The weeks of bargaining to put the November agreement in force were more difficult than anticipated, with one brief walkout by Iranian envoys and rancor among the bloc of nations that negotiated the deal. Russia and China, long Iran's protectors at the United Nations, pushed the United States to accept technical concessions that further make clear that Iran will retain the ability to enrich uranium, a key Iranian demand, once a final set of restrictions on its program is approved.
The Obama administration has preferred to blur that point in public, while arguing in private that the enrichment will be a face-saving token that does not pose a threat.
President Barack Obama welcomed the technical agreement, but he said a full, permanent deal will require further hard bargaining. That negotiation will take place even as the Obama administration appears likely to lose a fight to stop Congress from approving additional economic penalties on Iran.
The White House argued again Sunday that the imposition of further sanctions could ruin what might be the world's best chance to resolve the Iranian nuclear problem. Obama stressed that he would veto any legislation enacting new sanctions.
Congressional backers of additional sanctions argue that the threat of such measures would strengthen Obama's hand in the difficult talks ahead.
Israel, which considers Iran a mortal enemy, has warned that it will blow up Iranian nuclear development sites if diplomacy fails to blunt the threat of a nuclear attack by Iran on the nearby Jewish state.
The European Union's foreign policy chief, Catherine Ashton, said Sunday that the next step in the process is verification of the agreement by the U.N. nuclear watchdog agency.
"The foundations for a coherent, robust and smooth implementation of the joint plan of action over the six-month period have been laid," said Ashton, who led the negotiations for the bloc comprising the six powers.
The bloc had negotiated on and off with Iran for years, but there had been no visible sign of progress until the election last year of reform-minded Hassan Rouhani as the country's president.
Rouhani's election is widely seen as a recognition by Iran's supreme leader, Ayatollah Ali Khamenei, that the crushing sanctions and diplomatic isolation imposed on Iran over its nuclear program were not worth the cost.
Khamenei has given Rouhani and Foreign Minister Mohammad Javad Zarif greater license than many in the West had expected in reaching a deal. That includes a concession by Iran to stop enriching uranium to 20 percent potency, a level considered within striking distance of bomb-quality fuel. The existing stockpile of 20 percent fuel would be degraded over the six-month period of talks.
Iran will get about $4.2 billion in relief from the release of previously frozen oil revenue in that period. It will receive the first installment — $550 million — on Feb. 1, a senior U.S. official told the Washington Post on Sunday, speaking on the condition of anonymity because the arrangement was discussed privately.
Existing international sanctions will be paused but not revoked, which the United States, Britain and other partners say allows for a quick tightening of the economic noose if Iran balks. Skeptics in Congress and elsewhere say that once loosened, the oil sanctions will be impossible to revive.