WASHINGTON — Iran and six world powers took a significant and hard-won 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.

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 Obama welcomed the technical agreement, which will take effect Jan. 20, 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.

“Unprecedented sanctions and tough diplomacy helped to bring Iran to the negotiating table,” Obama said in a statement. “Imposing additional sanctions now will only risk derailing our efforts.”

The president 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. Republicans, in particular, accuse the White House of being cowed by what may be an Iranian bluff to walk away if new sanctions are approved. The White House, however, argues that although sanctions forced Iran to the table, the strategy has run its course.

Sen. Mark Kirk, R-Ill., scoffed at that rationale.

“Beginning Jan. 20th, 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 said in a statement. “I am worried the administration’s policies will either lead to Iranian nuclear weapons or Israeli air strikes.”

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 five permanent members of the U.N. Security Council — the United States, Britain, France, Russia and China — plus Germany.

The six-member 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, who is likely to make the final decision about developing any nuclear weapon, 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 said Sunday 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.

Zarif has said that if new sanctions are passed, “the entire deal is dead.”

Iran’s senior nuclear negotiator, Abbas Araghchi, said in an interview with Iran’s state news channel that the Islamic republic would continue enriching uranium to the 20 percent threshold until the last possible moment before Jan. 20.

Secretary of State John Kerry emphasized the positive in a statement Sunday.

“I want to commend the very critical and significant step today taken towards reaching a verifiable resolution that will prevent Iran from obtaining a nuclear weapon,” Kerry said.