The State Department confirmed that Rob Malley, the new pointman on Iran named by President Joe Biden's administration, was traveling to Vienna to lead the US delegation but that he did not expect to meet with his Iranian counterpart.

The meetings starting Tuesday aim to break a logjam as Biden supports a return to the 2015 nuclear deal but has insisted that Iran first reverse nuclear steps it took to protest sweeping sanctions imposed by former US president Donald Trump when he walked out of the accord.

"There's no denying that we are approaching this with urgency," State Department spokesman Ned Price told reporters, saying the "breakout time" if Iran decides to pursue a nuclear bomb had diminished under Trump.

Price reiterated that the United States was ready to look at lifting sanctions -- but only those related to the nuclear issue.

"We certainly will not entertain unilateral gestures or concessions to induce Iran to a better place," Price said.

"The original formulation is one that still holds today -- it's the limited lifting of nuclear sanctions in return for permanent and verifiable limits on Iran's nuclear program," he said, referring to the accord's text.

Upon exiting the agreement, Trump imposed sweeping sanctions including a US ban on any other country buying Iran's oil, a crucial export for the country.

Successive US administrations have imposed other sanctions including on human rights grounds that have also irritated Iranian leaders, but Washington has not put those on the table in nuclear talks.

Britain, China, France, Germany and Russia are also participants in the deal and are eager to see the return of the United States.

Iran said the meeting of the so-called 4+1 countries was to "talk about the path of lifting sanctions."

"Whether the joint commission's agenda produces a result or not depends on the Europeans and the 4+1 reminding the US of its obligations and the Americans acting on their commitments," Iran's foreign ministry spokesman Saeed Khatibzadeh said in Tehran.

"How and where the 4+1 talk to the US is their own business," he told reporters in Tehran.

The European Union has said its mediator will hold "separate contacts" with the United States in Vienna.

According to a senior EU official, two groups of experts from the other countries will work simultaneously, with one focused on US sanctions and the other on reinstating Iran's suspended nuclear commitments.

Khatibzadeh said experts from the Iranian delegation would explain "how (we plan) to stop our remedial measures."

"We have only one step, not step-by-step, (which) includes the lifting of all US sanctions," he stressed.

"It will become clear tomorrow whether the 4+1 can realize the points expected by Iran or not, so that we would have a clearer path forward," Khatibzadeh said.