Negotiations with Iran over its nuclear program will be extended for seven months, Secretary of State John KerryJohn Forbes KerryDemocrats debate how to defeat Trump: fight or heal GOP senators press State Department for Hunter Biden, Burisma records Krystal Ball hits media over questions on Sanders's electability MORE announced Monday, saying significant progress had been made in the final days.

It is the second extension, after an interim agreement was reached last year, for talks that seek to impose safeguards against Iran obtaining a nuclear weapon in exchange for the easing of sanctions.

The talks have faced skeptical lawmakers on both sides of the aisle, and the extension raises the prospect that Congress could act to strengthen sanctions.

Sen. Marco RubioMarco Antonio RubioGOP senators plan to tune out impeachment week Republicans warn election results are 'wake-up call' for Trump Paul's demand to out whistleblower rankles GOP colleagues MORE (R-Fla.), a potential 2016 presidential candidate, criticized the extension and pointed to the release of some frozen Iranian funds that began under the interim deal and appear set to continue.

“Papering over what appear to be significant differences and once again extending talks with Iran will not achieve the goal of a nuclear-weapon-free Iran," Rubio said in a statement. "During the course of these negotiations, Iran has received billions of dollars in sanctions relief, bolstering its economy."

I will continue working in the Senate to significantly increase the pressure on Iran on all fronts — nuclear, terrorism and human rights — before it’s too late," he added.

Iran and the six world powers in the negotiations will seek the broad outlines of a deal within four months, with the final details facing a seven month deadline.

"Progress was indeed made on some of the most vexing challenges that we face and we now see the path toward potentially resolving some issues that had been intractable," Kerry told reporters in Vienna, where the talks took place, in justifying why there is hope for a deal with the extension.

Kerry touted steps under the interim deal, saying Iran no longer has 20 percent enriched uranium, has frozen work on a nuclear reactor at Arak, and has allowed international inspectors access. He said the restraints on Iran remain in place during the extension.

"We would be fools to walk away from a situation where the breakout time has already been expanded rather than narrowed, and where the world is safer because this program is in place," Kerry said.

To the skeptics, Kerry said, "We have earned the benefit of the doubt," pointing to the interim agreement and saying it has "worked."

He said he is in close touch with members of Congress, including conversations while he was in Vienna.  

"I hope they will come to see the wisdom of leaving us the equilibrium for a few months to be able to proceed without sending messages that might be misinterpreted and cause miscalculation," he said of Congress.

— This story was updated at 12:52 p.m.