Iran and six world powers will continue negotiating toward a nuclear deal past Tuesday's deadline, and are now planning to have talks completed by Friday.

Senior State Department adviser Marie Harf said additional time is needed to reach a comprehensive long-term nuclear agreement.

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"We've made substantial progress in every area, but this work is highly technical and high stakes for all of the countries involved," she said in a statement. "We're frankly more concerned about the quality of the deal than we are about the clock, though we also know that difficult decisions won't get any easier with time — that is why we are continuing to negotiate."

"To allow for the additional time to negotiate, we are taking the necessary technical steps for the measures of the Joint Plan of Action to remain in place through July 10," she added.

By missing Tuesday's deadline, the already controversial deal faces a steeper battle in Congress. Had President Obama presented an agreement to Congress by this Thursday, lawmakers would have had only 30 days to review the terms. Since it won't make it to Capitol Hill in time, lawmakers will now have 60 days to review, giving opponents more time to find problems with the agreement.

Talks between Iran and the P5+1 countries — the United States, Britain, France, Germany, Russia and China — were widely expected to miss Tuesday's deadline for agreement, even though negotiators worked into the night Monday and continued to discuss the outstanding issues.

Though negotiators said they were close, news emerged Monday that Iran has demanded that the U.N. arms embargo be lifted as part of any deal, along with sanctions aimed at the country's ballistic missile program. Negotiators from the United States have told reporters in Vienna that this is a deal-killer, and lawmakers in Washington made it clear they would see it that way as well.

Today's extension is the second in as many weeks. The June 30 deadline for a long-term agreement was missed by negotiators and extended to Tuesday. But despite this latest extension, officials insisted that no decision was made to extend the deadline.

"We are continuing to negotiate for the next couple of days. This does not mean we are extending our deadline," said EU foreign policy chief Federica Mogherini said from a hotel in Vienna, where the talks are taking place, according to Reuters.

The extension comes as most Republicans continue to criticize the Obama administration's effort to reach a deal with Iran, which they say is misguided given the broken promises from Iran in the past. Some GOP lawmakers have said Obama needs to walk away from the deal if a good one cannot be reached, and Obama himself has said he would not strike a deal unless it's good.

"Tehran is seeking to negotiate a license to kill," House Foreign Affairs Chairman Ed Royce said Monday. "The supreme leader's negotiators have clearly been emboldened by the administration's concessions across the board. I can only hope that this is one line that U.S. negotiators aren't willing to erase."

The agreement taking shape would essentially freeze Iran's nuclear program for 10 years in exchange for relief from international sanctions that had crippled Iran's economy, and the release of more than $100 billion in frozen assets.

Negotiators have continued to extend the terms of the first interim deal, struck in November 2013, as they move toward finally reaching a final comprehensive one.

Russian Foreign Minister Sergei Lavrov said there was "every reason" to believe a deal would be done within "a few days."