The US secretary of state, John Kerry, warned Congress on Wednesday against hurting a historic opportunity for a nuclear pact with Iran by pressing ahead with new sanctions while international negotiators seek to prevent Tehran from being able to assemble an atomic weapons arsenal.

Kerry said the United States and other world powers are united behind an offer they presented to Iranian negotiators in Geneva last week. But he said new action now from US lawmakers could shatter an international coalition made up of countries with interests as divergent as France, Russia and China, endangering prospects for a peaceful end to the decade-long nuclear standoff with the Islamic republic.

The countries worry that Tehran is trying to assemble an atomic weapons arsenal. Iran insists its programme is solely for peaceful energy production and medical research.

"We put these sanctions in place in order to be able to put us in the strongest position possible to be able to negotiate. We now are negotiating," Kerry told reporters ahead of testifying before the Senate banking committee. "And the risk is that if Congress were to unilaterally move to raise sanctions, it could break faith in those negotiations, and actually stop them and break them apart."

With nuclear negotiations set to resume in Switzerland next week, the Obama administration dispatched Kerry and the vice-president, Joe Biden, to Congress on Wednesday to seek more time for diplomacy. They faced scepticism from members of Congress determined to further squeeze the Iranian economy and wary of yielding any ground to Iran in the talks.

Kerry said the negotiators should have a few weeks more to see if they can reach an agreement.

"The Iranian regime hasn't paused its nuclear programme," said Ed Royce, a Republican and House foreign affairs committee chairman.

"Why should we pause our sanctions efforts as the administration is pressuring Congress to do?"

Kerry said the potential accord with Iran relates to a "tough proposal," adding: "If it weren't strong, why wouldn't Iran have accepted it yet?"

Kerry said, "What we are asking everyone to do is calm down, look hard at what can be achieved and what the realities are. If this doesn't work, we reserve the right to dial back the sanctions. I will be up here on the Hill asking for increased sanctions, and we always reserve the military option. So we lose absolutely nothing, except for getting in the way of diplomacy and letting it work."

"We ought to be actually ratcheting up the sanctions against Iran," Republican Senate minority leader, Mitch McConnell.

"What the administration was promoting is something the Israelis think is a bad deal for them. It's pretty clear the Sunni Arab allies of ours also think it's a bad deal. Looking at it strictly from an American point of view, I think it's a bad deal as well."

Last week's talks broke down as Iran demanded formal recognition of what it calls its right to enrich uranium, and as France sought stricter limits on Iran's ability to make nuclear fuel and on its heavy water reactor to produce plutonium, diplomats said.

Barack Obama spoke on Wednesday with the French president, François Hollande. The two countries are in full agreement on Iran, the White House said in a statement.

Obama is under pressure at home and abroad to resolve the Iran nuclear standoff, having said the US has until some time next year before the Islamic republic could reach nuclear weapons capacity.

Obama has reached out in an unprecedented manner to Iran's new president, Hassan Rouhani, with the two men holding the first direct conversation between US and Iranian leaders in more than three decades.

At the same time, Obama has angered US allies Israel and Saudi Arabia, which see an Iranian nuclear arsenal as existential threats.

The new sanctions were overwhelmingly approved by the Republican-led House in July. The legislation blacklisted Iran's mining and construction sectors and committed the US to the goal of eliminating all Iranian oil exports worldwide by 2015. If the Senate banking committee pushes off its parallel bill any longer, lawmakers could attach it to a Senate defence bill, which could come up for debate as early as Thursday.