Diplomats from Iran and the US are to meet again in Switzerland this week as the pace intensifies for a nuclear deal with Tehran, US officials said Wednesday.

Top US negotiator Wendy Sherman will travel to Zurich to meet with Iranian officials on Friday and Saturday, less than a week after leading face-to-face negotiations in Geneva, the State Department announced.

US Secretary of State John Kerry, who met last week in Geneva and then again in Paris with his Iranian counterpart Mohammad Javad Zarif, will also be in Switzerland, attending the World Economic Forum in Davos, but it was unclear if they would hold another round of talks.

Kerry and his staff appealed Wednesday to US lawmakers to hold off on threatened new sanctions, which US President Barack Obama has already vowed to veto.

"The key to our negotiations is that whatever is agreed upon will show people with clarity that ... the path to a nuclear weapon is not achievable or has been given up or both," Kerry told reporters after meeting with EU foreign policy chief Federica Mogherini.

New US sanctions could threaten the unity of the global powers negotiating with Iran, he warned, saying "the United States acting unilaterally is not always the best path to take."

"This could all fall apart, including the sanctions regime. You lose the sanctions all together," Kerry warned.

Some lawmakers have argued that any deal between global powers known as the P5+1 and the Islamic Republic to curtail its nuclear ambitions in return for easing crippling sanctions should first be put to Congress for approval.

"The more I hear from the administration and its quotes, the more it sounds like talking points that come straight out of Tehran," said Senator Robert Menendez, a Democratic co-author of deferred-sanctions legislation that could soon be introduced in the Senate.

Senate Foreign Relations Committee chairman Bob Corker however has proposed adopting a bill that would require Obama to submit any final deal with Iran to Congress for final approval.

But Kerry argued: "It's very important to allow the executive branch of government, which in our constitution has the right to be able to do this negotiation, to do it.

"Then there's plenty of time for people to make judgements about how they feel about it."

Under an interim deal agreed in November 2013, Iran has frozen its uranium enrichment in exchange for limited sanctions relief. But two deadlines for a full accord cutting off Iran's pathway to an atomic bomb have been missed.

The P5+1 is now hoping to reach a "political understanding on the elements of a deal" by March 31, said State Department spokeswoman Jen Psaki.

That would leave several weeks to work out the technical details before a new June 30 deadline for a full agreement, she said.

"We're asking people to be responsible here," Kerry said, after his new deputy secretary of state Tony Blinken told lawmakers earlier that there was "a credible chance" of a deal.

"I think we've earned frankly a little credibility in this process ... and I think we ought to be able to proceed, on a careful basis, to complete these negotiations without any interference."