The US State Department played down expectations for indirect talks that begin on Tuesday in Vienna on Washington and Tehran resuming compliance with the 2015 Iran nuclear deal, saying it expected the talks to be difficult.
"We don't underestimate the scale of the challenges ahead. These are early days. We don't anticipate an early or immediate breakthrough as these discussions, we fully expect, will be difficult," State Department spokesman Ned Price told reporters during his daily briefing on Monday.
Earlier, Iran said the outcome of the Vienna talks will depend on European parties leaning on the US to lift sanctions.
Britain, France and Germany are pushing to bring US President Joe Biden's administration back into the nuclear deal, after his predecessor Donald Trump's withdrew and reimposed sanctions on Iran.
The other participants, including China and Russia, are to meet Tuesday in-person in the Austrian capital, with the United States indirectly taking part.
"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.
"How and where the 4+1 talk to the US is their own business," he told reporters, noting there will be no negotiations in Vienna.
The meeting's aim is to "talk about the path of lifting sanctions", he added.
The deal, also known as the Joint Comprehensive Plan of Action (JCPOA), promised Iran relief from international sanctions in exchange for limits on its nuclear program.
Biden has promised to rejoin the agreement on condition that Iran first returns to nuclear commitments it suspended in response to reimposed sanctions.
Tehran, however, says Washington has to end sanctions first.
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 rolling back Iran's suspended nuclear commitments.
Khatibzadeh said experts from an 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.
The Vienna meeting comes after a video conference of a JCPOA joint commission held on Friday.