European Union foreign policy chief Federica Mogherini says that Iran nuclear talks in Vienna will continue past the July 7 deadline in the hope a deal can be finalized. (Reuters)

Nuclear talks between Iran and world powers pushed past their latest deadline Tuesday as negotiators insisted that a deal could be reached by the end of the week but said they remain ready to walk away if agreement proves impossible.

“I believe we will in the near term either get this deal or find out we can’t,” a senior Obama administration official here said. “On any given day, if we feel that we’re just not going to get there, that will be that.”

Negotiators hesitated to call their new goal of completing an agreement by Friday a deadline. But a State Department official said an interim accord that has paused and reversed elements of Iran’s nuclear program since early last year would be extended at least until then.

Foreign ministers of U.S. negotiating partners Britain, France, Germany, Russia and China — who arrived here Monday in the hope of sealing a deal — left Vienna again late Tuesday, saying they would return later in the week.

Secretary of State John F. Kerry and Iranian Foreign Minister Mohammad Javad Zarif remained, continuing the grueling and nearly nonstop meetings that they and their aides have conducted for the past 11 days on what they have called the “endgame” of more than 18 months of negotiations.

U.S. Secretary of State John F. Kerry and U.S. Undersecretary for Political Affairs Wendy Sherman meet with foreign ministers of Germany, France, China, Britain, Russia and the European Union in Vienna on July 7, 2015. (Pool photo by Carlos Barria/AFP/Getty Images)

Zarif said he had canceled plans to accompany Iranian President Hassan Rouhani on Thursday to Russia for a summit of major emerging nations. “Unfortunately, I have to stay in Vienna for nuclear talks,” Zarif told reporters.

Negotiators were mostly tight-lipped on the remaining sticking points in any deal to lift international sanctions against Iran in exchange for long-term restrictions aimed at preventing Iran from being able to build nuclear weapons. Iran has insisted that its nuclear activities are purely for peaceful energy and research purposes.

Iranian officials here, and Russian Foreign Minister Sergei Lavrov, said one of the most vexing remaining issues was whether the U.N. Security Council would lift prohibitions on sales to Iran of conventional weapons and ballistic missiles.

Iran wants the bans lifted as part of an agreement, but the United States and others say elements of those sanctions are separate from the talks. “There will be continued restrictions in this area,” the senior administration official said.

French Foreign Minister Laurent Fabius, as he prepared to return to Paris, said that “we are insisting especially on necessary limitations on [Iran’s] nuclear research and development,” as well as sanctions and a full accounting of Iran’s past nuclear work.

The administration official appeared more optimistic about some of those issues. “In order for us to have a good deal, we have to be satisfied on the access question. I believe we will be,” the official said, referring to inspections by the International Atomic Energy Agency of Iran’s current nuclear facilities and the provision of information about its past nuclear programs.

White House press secretary Josh Earnest says “significant differences” remain between negotiators in the Iran nuclear talks but that the the process is “worth continuing.” (Reuters)

The official, who briefed reporters on Tuesday’s events on the condition of anonymity, said the U.S. side was also confident about how Iran plans to expand its nuclear development when the agreement expires, in stages of 10, 15 and 20 years.

“We know where Iran plans to head,” and the agreement will ensure “appropriate development of their program,” the official said, adding that the IAEA would determine whether the specifics of its verification agreements with Iran would be made public.

The extension of the talks, assuming an agreement eventually is reached, could complicate the administration’s effort to convince skeptical U.S. lawmakers, returning Tuesday from their Fourth of July recess, that the deal is a good one.

Under compromise legislation passed overwhelmingly in May, President Obama agreed to a 30-day legislative review period before he would use his executive power to waive any U.S. sanctions on Iran as part of the deal. The review period grows to 60 days if the text of an agreement is not sent to Congress by Wednesday, a deadline that is sure to be breached.

Sen. Bob Corker (R-Tenn.), the principal author of the legislation, said Tuesday he was afraid that U.S. negotiators had been unduly motivated to meet the Wednesday deadline to avoid an extended congressional review. “I think that people would rather them slow down based on the way that it’s going,” he said, adding that he had relayed that sentiment to Kerry in a recent conversation.

Administration officials have said they think that if the agreement is sellable to Congress, it will not matter when lawmakers receive it.

“We welcome additional scrutiny of the deal,” White House press secretary Josh Earnest said Tuesday. “If we are able to reach one, we’re confident it will clearly be in our interest, one that clearly shuts down every pathway Iran has to a nuclear weapon.”

Earnest placed responsibility for failure thus far on Tehran, saying that “the Iranians so far have not been able to sign on the dotted line of a final agreement that reflects the broad parameters” that negotiators set in a framework deal in early April.

Whichever party is responsible for the delay, the administration official said that time is the enemy of all involved. “Everyone understands that time does not help get those decisions made,” the official said. “As difficult as it might be for the Americans to go home and deal with the politics” of failure, “it’s pretty darn hard for the Iranians.”

Speaking of the possibility of a formal break in the talks, and the decision by Kerry and Zarif to remain in Vienna, the official said that “once we leave here, we are in less control of what happens in this negotiation. It gets more complicated, not less.”

David Nakamura and Mike DeBonis in Washington contributed to this report.

Read more:

Here’s what is in the way of a ‘good’ Iran nuclear deal

The key moments in the long history of U.S.-Iran tensions

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Today’s coverage from Post correspondents around the world