Top Obama administration officials have been pushing U.S. lawmakers hard to hold off on new sanctions over Iran's nuclear program, Reuters reported on Wednesday.
Some key lawmakers, however, have clarified they are not yet convinced to support a delay.
Senator Bob Corker, the top Republican on the Senate Foreign Relations Committee and a member of the Senate Banking Committee, which is considering the sanctions package, said lawmakers were skeptical because they felt they had to push the White House to back strict sanctions on Tehran.
"It's incumbent upon them over the next 24 to 48 hours to persuade folks like me and others that the course of action they want to follow is a sound one," Corker told Reuters.
"I think ... because Congress had to push the administration into the sanctions regime in the first place, there is a degree of skepticism. But from my standpoint I'm certainly open to listening," he added.
Corker met on Wednesday with Secretary of State John Kerry. On Thursday, Kerry and Treasury Secretary Jack Lew were to hold a classified briefing on the status of talks with Iran for the Senate banking panel.
"The point that they are making is that they are at a point in these negotiations where they believe that additional sanctions coming out of the committee are counterproductive to the negotiations that are under way," Corker said.
The banking panel had been expected to vote on the sanctions in September, but held off after the Obama administration asked for a pause while negotiations with Tehran got under way.
Senator Mark Kirk, a Republican member of the banking panel who is a strong backer of tougher sanctions, said he opposed any further delay.
"Every day the Senate delays consideration of new sanctions, Iran installs more centrifuges, enriches more uranium and improves its nuclear breakout capability," he said, referring to the ability to enrich uranium for use in a bomb.
"If Iran is capable of negotiating while violating international law, the United States should be equally capable of negotiating while imposing new sanctions pressure," Kirk said in a statement quoted by Reuters.
Members of both the Republican and Democratic parties have overwhelmingly backed tougher economic pressure on Iran in recent years amid concern it is closing in on nuclear weapons capability.
Two weeks ago, six Democratic and four Republican senators called on Iran to end all its uranium enrichment activity and pushed for a speedy escalation of sanctions against the Islamic Republic.
In July, the House of Representatives overwhelmingly passed a bill blacklisting Iran’s mining and construction sectors.
While Senators have called to toughen sanctions, President Barack Obama has welcomed the moderate statements made by Iran’s new president, Hassan Rouhani, and has said he wants to test their sincerity.
Iranian negotiators recently met in Geneva with representatives of the so-called P5+1 - the United States, China, Russia, France, Britain and Germany. During the two-day session, Iran presented what it described as a breakthrough proposal that would include snap inspections of its atomic sites.
The proposal was described by the White House as "useful". White House spokesman Jay Carney said it showed a "level of seriousness and substance that we have not seen before." Western negotiators described the talks as the most detailed and serious to date.