BERLIN — Iran said initial talks Tuesday in Vienna on returning to the 2015 nuclear deal were "constructive" as mediators shuttled between Iranian and American envoys seeking a road map to lift U.S. sanctions and bring Tehran back to its commitments under the accord.

Iran's lead negotiator, Deputy Foreign Minister Abbas Araghchi, characterized the initial talks as on "the right track."

"It's too soon to say it has been successful," he added during an interview with Iran's Press TV, reiterating the Iranian demand that U.S. sanctions be lifted in one step.

A U.S. team led President Biden's special envoy for Iran, Robert Malley, was in the Austrian capital for the discussions — which are expected to continue on Friday — but did not meet directly with Iran, setting up camp in a different hotel.

For the United States and others, the core issue is having Iran scale back its uranium enrichment to levels outlined in the nuclear deal. Iran wants an end to sanctions placed by the Trump administration after withdrawing from the accord in 2018.

But the two sides have been at loggerheads over who takes the first step.

Enrique Mora, the European coordinator for the talks, tweeted that he would "intensify separate contacts" with all relevant parties, including the United States.

The Trump administration walked away from the deal — designed to curb Iran’s nuclear program — and imposed hundreds of sanctions and restrictions that are expected to complicate U.S. efforts to return to compliance.

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Iran complained it could not reap the economic benefits of the deal as promised, and it gradually breached its commitments, notably by ramping up uranium-enrichment levels and limiting inspections.

Backers of the pact see an urgency to get it back on track: the dwindling breakout time before Iran is expected to be able to produce enough fissile material for a potential nuclear weapon, alongside approaching elections in Iran that may usher in a more hard-line government less inclined toward diplomacy. Iran has said for decades it does not want nuclear weapons and seeks enrichment uranium for reactors.

The talks are an important goal for Biden, who campaigned on a pledge to return to the agreement. But the timing and structure of the meetings represent a disappointment for those who hoped for a quick and muscular U.S. engagement with Iran.

Approaching three months in office, Biden has not made any bold gesture to rejoin the deal, and the United States and Iran remain publicly at odds. Biden rarely mentions the deal unless asked about it directly.

The back-burner approach stands in contrast to Biden’s fast pace in offering legislation and initiatives to address domestic priorities, including blunting the coronavirus pandemic and its economic fallout.

Biden’s advisers also appear split about whether rejoining the deal is the best way to contain Iran’s nuclear program, especially since some of its 2015 provisions begin to expire soon. Biden has said he wants to leverage the existing deal to get a broader and stronger agreement, but Secretary of State Antony Blinken and others have not ruled out other options.

Iran has pushed back at suggestions of renegotiation or expansion, insisting that Washington instead return to the deal it signed.

Biden, meanwhile, has not budged from a hard public line that Iran must stop nuclear activities that violate the deal before the United States would drop sanctions that Trump reimposed.

But European diplomats say they will negotiate a list of moves for each side in parallel, to overcome the arguments over which side acts first. The plan is for Tehran and Washington to then implement the steps in coordination.

In their initial meeting Tuesday, two expert groups were formed to identify “concrete measures to be taken by Washington and Tehran to restore full implementation of the JCPOA,” Russian ambassador to Vienna Mikhail Ulyanov tweeted. But the process is expected to be a slow one.

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Sometimes-overlapping layers of economic sanctions brought in during the Trump administration’s shift to a campaign of “maximum pressure” on Tehran make the task ahead particularly complex, said Thomas Countryman, former acting undersecretary of state for arms control under the Obama administration.

“Both Trump administration officials and the regime-change lobby here in Washington were and still are very explicit that the purpose was to make it as difficult as possible for a successor administration to remove sanctions — to make it difficult both bureaucratically and politically,” he said.

White House press secretary Jen Psaki would not predict how long the talks would run or whether they can be concluded ahead of elections in Iran in June that may change the landscape for Iranian participation.

The indirect structure, with European powers serving as go-betweens, “is still, in our view, a step forward toward diplomacy, and that remains our first objective,” Psaki said.

“We certainly expect that the primary issues that will be discussed over the course of the coming days are the nuclear steps that Iran would need to take to return to compliance with the terms of the [nuclear deal] and the sanctions relief steps that the United States would need to take to return to compliance, as well,” Psaki said.

State Department spokesman Ned Price also played down expectations for a swift breakthrough.

“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. But we do believe that these discussions with our partners and, in turn, our partners with Iran is a healthy step forward,” he said.

Gearan reported from Washington and Fahim from Istanbul.