U.S. Secretary of State John Kerry is telling lawmakers to "calm down" and give negotiators a chance to resolve deep concerns over Iran's nuclear program diplomatically.
Kerry briefed lawmakers with the Senate Banking Committee Wednesday in Washington, saying this is not the time to step up the pressure on Tehran.
"We have all the opportunity of negotiations to make certain this is a peaceful program that can threaten nobody," he said. "And what we're asking everybody to do is to calm down, look hard at what can be achieved, what the realities are."
Kerry's comments, made to reporters before the closed-door session, came as a number of lawmakers called for hitting Iran with new economic sanctions, saying Iran's reported concessions on uranium enrichment and inspections are not enough.
Kerry said last week's talks in Geneva are just a start, and that there should be no rush to judgment.
"We believe we have put ourselves into the strongest position to start [negotiations with Iran]. This is a first step, not a final agreement," he said.
Iranian officials are asking for relief from economic sanctions that have devastated its economy while insisting Iran has a right to enrich uranium.
Senators like Republican Banking Committee member Bob Corker say, though, that so far the United States is not being firm enough.
"All of us want to see this dealt with diplomatically. A diplomatic solution is best," he said. "We also know that if you negotiate a weak solution it also leads you to a very terrible place with either Iran having a nuclear weapon or us being involved in military activity."
Meanwhile, Republican Senator John McCain criticized the Obama administration's approach to the talks, saying it has hurt Washington's credibility.
"This whole session of negotiating is a fiasco and allows the Iranians to continue to enrich," he said. "And thank God for the French and there's no possible way this proposal should meet any form of agreement."
Democratic Senator Carl Levin disagreed, saying adding more sanctions now could backfire.
"I support maintaining tight sanctions against Iran," he said. "I don't support increasing them at this time because I think it could interfere with the negotiations and, I think, it could lose some of the support we have for sanctions, particularly with China and Russia.,"
At the White House Wednesday, spokesman Jay Carney said while strong sanctions had brought Iran to the negotiating table, it was now time to let diplomacy take over.
"They [the Iranians] are engaging in serious negotiations about how they can, verifiably, comply with their international obligations in a way that allows the P5+1 and all of our allies and partners and every country in the region and the world to be confident that Iran cannot and will not obtain a nuclear weapon," he said. "That is the goal."
But many lawmakers remain unconvinced. At a hearing earlier Wednesday, House Foreign Affairs Committee Chairman Republican Ed Royce questioned the benefits of a proposed deal.
"There is growing concern in Congress that the outlines of this agreement do not meet the standards needed to protect the United States and to protect U.S. allies," he said.
The committee's ranking Democrat, Eliot Engel, also warned that Iran needs to give more or risk facing new sanctions.
"If these talks are about Iran abandoning its nuclear program, then to show good faith, at the very least while the talks are going on, Iran should stop enrichment, period," he said.
Iran insists its nuclear program has always been designed for peaceful purposes and is seeking an easing of sanctions as part of any deal. The State Department has warned enacting new sanctions at this stage would be a mistake, urging lawmakers to give diplomacy more time.
Officials from both Iran and the P5+1 group, made up of the United States, Britain, France, China, Russia and Germany, have expressed optimism about their latest round of negotiations, despite failing so far to agree on a deal. The talks will continue next week.
Iran also has been negotiating with the United Nations' nuclear agency and recently signed an agreement that will allow expanded inspections of its nuclear sites. The head of the International Atomic Energy Agency says he sees "no radical change'' in Iran's nuclear program over the past three months.
Yukiya Amano told Reuters Wednesday Tehran has continued its most sensitive nuclear activity, enrichment of uranium to 20 percent. Iran says it needs the 20 percent material to fuel a medical research reactor. Experts warn it puts Iran on the fast track to producing 90 percent enriched uranium required for making a nuclear warhead.