Iran and the US will hold indirect talks in Vienna on Tuesday aimed at bringing both countries back into full compliance with the 2015 nuclear deal that Washington abandoned three years ago.
Iran has steadily overstepped the accord's limits on its nuclear program in response to Washington's withdrawal from the accord in 2018 and its reimposition of sanctions that have crippled the Islamic Republic's economy.
While Tehran has repeatedly rebuffed "direct and indirect negotiations" with its long-time foe, Washington said on Monday it expected indirect talks with Iran about reviving the deal to be difficult.
Both Tehran and Washington did not foresee any early breakthrough.
A Western diplomat told Reuters on Friday that a shuttle-diplomacy approach would be adopted in Vienna.
"What other parties to the deal do is their business. Where and how they negotiate with the United States is up to them. The Iranian delegation will not have any talks with the US delegation in Vienna at any level," Iranian media on Tuesday quoted Foreign Ministry spokesman Saeed Khatibzadeh as saying.
Adding fresh doubt to the chances of a breakthrough on Tuesday, an Iranian official told Reuters: "Our agenda during the meeting (in Vienna) will be removal of all US sanctions against Iran ... as our supreme leader has said repeatedly, anything less than that will not be accepted by Tehran".
Iran's Supreme Leader Ayatollah Ali Khamenei, who has the last say on all state matters, has opposed any gradual easing of sanctions.
President Joe Biden's administration wants to revive the accord but has said that this requires negotiations. Tehran has dismissed any engagement in talks with Washington about both sides resuming compliance with the deal.
"Iran will reverse its nuclear steps only after all US sanctions on Iran are lifted. After verifying it, which will take a few hours, Iran can quickly reverse its nuclear steps," the official said.
The US State Department has said the focus of the Vienna talks will be on "the nuclear steps that Iran would need to take in order to return to compliance" with the nuclear accord.
Under the 2015 accord, US and other economic sanctions on Tehran were removed in return for curbs on Iran's nuclear program to make it harder to develop a nuclear weapon - an ambition Tehran denies.