WASHINGTON — Iran and world powers struck late Friday a deal to extend their Sunday deadline to strike a nuclear accord easing fears Tehran will get the bomb, negotiators in Vienna revealed. With the extension of talks, Iran committed to taking additional steps that US officials said would set back their progress toward a nuclear bomb, and will receive in return an additional $2.8 billion in assets that are currently frozen in overseas banks.

The High Representative of the European Union for Foreign Affairs and Security Policy, Catherine Ashton, and the Foreign Minister of the Islamic Republic of Iran, Mohammad Javad Zarif issued a joint statement in which they announced the four-month extension that would continue the talks until November 24.

“We, together with the Political Directors of the E3+3 (China, France, Germany, Russia, the United Kingdom and the United States), have worked intensively towards a Joint Comprehensive Plan of Action, building on the political momentum created by the adoption and smooth implementation by both sides of the Joint Plan of Action (JPA) agreed on 24 November 2013,” the two wrote.

They emphasized that the extension was within the timeframe initially delineated in the JPA, which set out a six month-long period for talks toward a comprehensive agreement, but allowed for a six-month extension.

Secretary of State John Kerry noted that during the initial six-month period, Iran had upheld its obligations under the JPA, including Since its implementation, neutralizing its stockpile of 20 percent enriched uranium; capping its stockpile of 5 percent enriched uranium; not installing advanced centrifuges; not installing or testing new components at its Arak reactor; and submitting to far more frequent inspections of its facilities.

Senior administration officials said Friday that under the extension, Iran will also make all of its 20 percent enriched uranium into fuel for the Tehran Research Reactor. In its fuel form, administration officials said, it will be very difficult for Iran to use this material for a weapon in a breakout scenario.

The officials also said that Iran has agreed that rotors for advanced centrifuges will now only be manufactured at plants that are under IAEA inspection, and that those rotors will only be used to replace damaged existing machines.

In return, an additional $2.8 billion will be released from the approximately $100 billion of frozen Iranian assets. This amount, officials said, was a prorated amount based on the initial $4.8 billion released in the first six months of the JPA.

Zarif and Ashton noted that over the past few weeks, since talks reconvened in early July, all parties “have further intensified our efforts” but added that “while we have made tangible progress on some of the issues and have worked together on a text for a Joint Comprehensive Plan of Action, there are still significant gaps on some core issues which will require more time and effort.”

This assertion was backed up by US officials, who specified that while progress had been made regarding the Arak heavy water facility and the formerly secret reactor at Fordo, gaps remain on key topics including research and development, domestic enrichment, and particularly enrichment capacity at the Natanz facility.

According to senior US administration officials, negotiators who have spent weeks in Vienna will now head home to their respective capitals, bringing with them a number of possible plans that they will have to review and consider before talks reconvene in late August.

Iran and the six powers have been negotiating almost constantly for months, trying to forge an accord by July 20 when an interim deal agreed in Geneva in November expires.

After a decade of rising tensions, the mooted accord between Iran and the five permanent members of the UN Security Council plus Germany is aimed at easing concerns that Iran might develop nuclear weapons and silencing talk of war.

Iran denies seeking the atomic bomb and wants the lifting of crippling UN and Western sanctions.

The six powers want Iran to dramatically reduce in scope its nuclear program for a lengthy period of time and agree to more intrusive UN inspections.

This would greatly expand the time needed for the Islamic republic to develop a nuclear weapon, should it choose to do so, while giving the world ample warning of any such “breakout” push.

Washington and Iran earlier this week laid the groundwork for pushing back the deadline after US Secretary of State John Kerry held two days of intense talks with his Iranian counterpart that failed to produce a breakthrough.

“It’s clear that we’ve made real progress in several areas and that we have a credible way forward, but as we approach a deadline under the interim deal, there is still significant gaps between the international community and Iran and we have more work to do,” US President Barack Obama told reporters on Wednesday.

The two sides are believed to have narrowed their positions in recent weeks on a few issues such as the Arak reactor, which could give Iran with weapons-grade plutonium, and enhanced inspections.

But they remain far apart on the key issue of Iran’s capacities to enrich uranium, a process which can produce fuel for reactors but also the core of a nuclear bomb.

The six powers want Iran to reduce dramatically the scope of its enrichment program, while Tehran wants to expand it.

The US and Iranian governments are both under intense domestic pressure not to give too much away.

Some US lawmakers have threatened to ramp up sanctions without a rigorous agreement – and a bill is still waiting in the wings in the Senate that would set an automatic trigger for tougher sanctions should talks fail. Senator Mark Kirk (R-IL), a sponsor of the additional sanctions legislation responded via Twitter to the Friday evening announcement, writing “more time for deal w/ #Iran = time to build nuclear bomb. Need non-military pressure via Menendez-Kirk sanctions NOW.”

Senator Robert Menendez (D-NJ), the other sponsor of the sanctions bill, has called for an accord that dismantles Iran’s nuclear program in a way that is verifiable for 20 to 30 years.

While stating their emphatic opposition to any additional sanctions legislation that they said might harm the diplomatic effort, senior administration officials echoed Menendez’s call for an agreement whose terms would extend “into double digits.”

AFP contributed to this report.