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 / Updated  / Source: NBC News
By Halimah Abdullah

Secretary of State of John Kerry speaking from Vienna stressed that while negotiators working to craft a nuclear deal with Iran would not continue working indefinitely, they are making progress toward a comprehensive deal.

And they will not feel rushed by deadlines.

"We believe we're making real progress," Kerry said. "But, as I've said many times and discussed with President Obama, we're not going to sit at the negotiating table forever."

The tempo of talks picked up on Thursday as negotiators from the U.S. and five other world powers continue to hammer out a framework with Iran aimed at keeping that country from developing nuclear weapons ahead of a third deadline for discussions.

Related: Iran Nuclear Talks: Kerry, Zarif Locked in Meetings as Deadline Looms

The nuclear deal with Iran is a key part of President Barack Obama's foreign policy legacy, a fact he underscored during an interview with The Atlantic.

Ahead of his public comments, Kerry had more than a half a dozen, one-on-one meetings or phone calls with foreign ministers on Thursday.

The administration also offered several public progress reports.

In a statement released Wednesday, the White House said President Obama met with Kerry and Secretary of Energy Ernest Moniz via a secure teleconference and the president “reviewed the progress of negotiations to date, and provided guidance related to our ongoing efforts to achieve a good deal between the P5+1 and Iran that meets our requirements.”

In a Tweet, Kerry said negotiators continue to “discuss difficult issues this AM. Working diligently to see if agreement possible.”

With negotiations at a critical stage, Obama told top Democratic senators he feels there is a less than “50-50” chance of securing a deal with Iran

“He said, in the course of negotiations he's been more optimistic, less optimistic, and he said that the chances at this point are below 50-50, this week,” Sen. Dick Durbin, D-Illinois, the number two Democrat in the senate and an Obama ally said of the president’s comments during a closed door meeting on Tuesday night.

Related: Kerry: Iran Negotiators Making Progress on Nuclear Deal

Durbin added the president’s comments to the senators came before he received an updated report from Kerry.

The current round of talks has blown through two deadlines already and has been extended until Friday, but the Obama administration must submit an agreement to Congress before Thursday turns to Friday in Washington if it wants to avoid an extended legislative review.

If the administration misses that target, the congressional review period will double from 30 to 60 days, possibly delaying the sanctions relief that the U.S. would have to give to Iran under the terms of an agreement.

Kerry spoke by phone with Russian Foreign Minister Sergey Lavrov, who is in Russia and voiced optimism, saying he was prepared to return to Vienna. Lavrov's deputy, Sergei Ryabkov, who is in Vienna, was quoted by Russian news agencies as saying that a deal could be reached rapidly but it would require "political will.”

Iranian officials said a deal is unlikely to come Thursday.

Iranian Foreign Minister Javad Zarif indicated that hard work remained and that Tehran would continue with talks.

"Mark my words; you can't change horses in the middle of a stream," Iranian Foreign Minister Javad Zarif Tweeted on Thursday.

The Associated Press contributed.