Iran's Supreme Leader Ayatollah Ali Khamenei on Thursday warned against "deceitful" world powers and branded a letter from Republican lawmakers as a sign of America's internal collapse just days before new nuclear talks.

Khamenei's broadside came as US Secretary of State John Kerry left Washington on a trip which will see him once again huddle with his Iranian counterpart in Switzerland aiming to seal a deal to curtail Tehran's nuclear program.

With the nuclear negotiations between Iran and world powers in the so-called P5+1 group now seen to be in the critical endgame, political tensions are soaring in the US among Republicans opposed to a rapprochement with America's old foe.

"I think we're all aware that the next couple of weeks are going to be important. They're vital. We're at the crunch time here," State Department spokeswoman Jen Psaki told reporters.

The White House has reacted angrily to the letter signed by 47 Republican senators insisting that any deal reached by the administration of President Barack Obama could be modified by Congress.

Kerry said that was "flat wrong" before he left, heading first to Egypt and then to Lausanne where he will meet from Sunday with Iranian Foreign Minister Mohammad Javad Zarif.

Khamenei however warned Iran's highest clerical body that the letter was a sign "the other party is deceitful and stabs in the back."

Iranian officials "know what they are doing and they also know how to act in case of an agreement so that Americans cannot break it later", said Khamenei, who will have the final say on any deal.

Khamenei also said the missive indicated "the extreme decadence of political ethics and the collapse of the American system from within," ISNA news agency reported.

After years of stop-start negotiations, global powers have been holding months of intense closed-door talks with Iran since Iranian President Hassan Rouhani came to office in August 2013.

An interim accord struck in November 2013 has caused the Islamic Republic to freeze much of its nuclear enrichment programme, in return for a slight easing of international sanctions which have damaged the country's economy.

- No further irritations -

As a March 31 deadline for a political framework looms, Kerry and other US administration officials denounced the Republicans' letter as undermining America's credibility not just with Iran, but with the other global powers involved in the talks -- Britain, China, France, Germany and Russia.

The letter allows Iran to cast doubt on the credibility of the West, German Foreign Minister Frank-Walter Steinmeier agreed at a Washington think-tank, adding "this is not a trifle."

"The negotiations are difficult enough, so we didn't actually need further irritations," Steinmeier said Thursday.

Zarif is also due to meet with European negotiators in Brussels on Monday, and is expected to then return to Lausanne for further meetings with Kerry.

Psaki refused to predict whether a deal could be reached on this Kerry trip, and recalled that the US secretary of state is due to return to Washington for meetings on March 23 and 24.

The Persian New Year, Nowruz, is also due to be celebrated in Iran around the same time.

"I think our expectation is that we will work 'til the end of March," Psaki told reporters.

Iran has always denied seeking to arm itself with a nuclear weapon, but global powers insist it must prove that its atomic programme is purely peaceful.

"Iran's path to the nuclear bomb must be blocked in an unambiguous, verifiable and durable way," Steinmeier said late Wednesday before dining with Kerry.