World powers struggled to settle on the terms of an agreement with Iran on Thursday as negotiations resumed in Geneva over the Islamic Republic's controversial nuclear program.

The United States entered the second formal round of talks with Germany and fellow permanent members of the United Nations Security Council— France, the United Kingdom, Russia and China— outlining the parameters of an interim deal with Iran that US President Barack Obama would find acceptable.

White House spokesman Jay Carney said on Thursday that any sanctions relief on the table would be reversible and emphasized that it would only move forward if Iran offered "concrete, verifiable measures" in return.

The relief would not affect the core sanctions architecture, Carney said, and could be reinstated as quickly as it could be eased by the Obama administration.

But upon news that the US would consider offering Iran sanctions relief, key members of the United States Senate decided not to wait until the end of the summit on Friday to move forward with a punishing new sanctions bill against Tehran.

Senator Tim Johnson, chairman of the Senate Banking Committee, said on Thursday that he already had the go-ahead from Senate Majority Leader Harry Reid to mark up the bill— the last procedural move required before the legislation hits the chamber floor for a vote.

Later, Johnson told the Jerusalem Post as clarification that he plans "to wait to hear any results of those talks from our negotiators before making a final decision."

Senator Robert Menendez, chairman of the Foreign Relations Committee, expressed confusion with the policy coming from his fellow Democrats in the White House for "unilaterally" offering sanctions relief without consulting Congress.

"What I do not understand is a negotiating posture in which we suspend our actions, we give them sanctions relief on existing sanctions, yet they continue to be able to enrich," Menendez said on Thursday.

On his way to a separate vote, Menendez "applauded" the move to markup "in the absence" of substantial concessions from Iran.

The development will likely complicate an already difficult negotiation in Geneva, with both sides struggling to find space to concede without angering their domestic political classes.

While Iranian officials said that they were hopeful a deal could be forged by the end of the summit, they cautioned that longstanding differences between Iran and the international community remain "tough" to bridge.

"I'm hopeful that we can move forward. We are making progress, but it's tough," Iranian Foreign Minister Mohammad Javad Zarif told Reuters. "The differences are widespread and deep. This is undeniable."

US Under Secretary of State Wendy Sherman, leading the American negotiating team, said after the meetings that the talks had been "substantive and serious" between the seven parties.

Sherman and Iranian Deputy Foreign Minister Abbas Araqchi held a one hour meeting on the sidelines of the Geneva talks.

Israeli officials consider the US offer of "limited," reversible sanctions relief in return for a pause of the Islamic Republic's most troubling nuclear activities too generous and vow to publicly oppose such a deal.

"Israel understands that there are proposals on the table in Geneva today that ease the pressure on Iran for concessions that are not concessions at all. The proposal would allow Iran to retain the capabilities to make nuclear weapons," Prime Minister Binyamin Netanyahu warned on Thursday during a speech to the Jewish Agency.

"Israel totally opposes these proposals. I believe that adopting them would be a mistake of historic proportions and should be rejected outright," Netanyahu said.

A senior Obama administration official said the US delegation in Geneva was in regular contact with their Israeli counterparts to keep them apprised of developments.

"It is crucial that we have this space to negotiate the final agreement without Iran’s nuclear program continuing to march forward," the US official told reporters in Geneva on Wednesday. "That is the purpose of a first step."

But Netanyahu doubled down against any agreement with Iran that stops short of getting it to halt its uranium enrichment, amid reports that just such a deal was in the works.

"The sanctions regime brought the Iranian economy to the brink of the abyss, and the policies of the P5+1 can compel Iran to fully dismantle its nuclear weapons program," Netanyahu said. "This means ending all enrichment and stopping all work on the heavy water plutonium reactor."

Netanyahu added that "anything less" would decrease the chances of reaching an agreement through peaceful means. "Israel always reserves the right to defend itself, by itself, against any threat," he asserted.

In a meeting with a delegation of US congressman, Netanyahu called the deal being discussed "the deal of the century" for Iran. He told the delegation that under the deal Iran practically gives up nothing, perhaps a "few days of enrichment," while "all the air" will be taken out of the sanctions regime that took years to build.

"This is a huge mistake that will relieve the internal pressure inside Iran," he stated.

Intelligence Minister Yuval Steinitz added in an Army Radio interview that any agreement needed to give Israel the "quiet" that the Iranians have neither enrichment nor plutonium reactor capabilities.

"We are against any interim agreement, because if we begin to dilute the sanctions, the whole sanctions regime will collapse," he said. "Even if it will not collapse, implementation of sanctions will be difficult if not impossible, the pressure on Iran will decrease, and an agreement with them will not be effective."

Steinitz said the deal being discussed is likely to be a "bad agreement, a historic mistake that will harm us all."

Please LIKE our Facebook page - it makes us stronger