U.S. and Iranian officials said Tuesday an initial day of talks in Vienna on returning to the 2015 nuclear deal were "constructive," but the Biden administration cautioned that no immediate breakthroughs were anticipated on one of the new president's top foreign policy goals.

The European-led diplomatic effort featured mediators shuttling between Iranian and American envoys, a far cry from the intensive face-to-face discussions held by U.S. and Iranian diplomats who brokered the original agreement.

The goal now is agreeing on a road map toward lifting U.S. sanctions that were imposed under President Donald Trump and recommitting Tehran to its agreements under the accord, a complicated undertaking with no guarantee of success.

Still, Iran's lead negotiator, Deputy Foreign Minister Abbas Araghchi, characterized the initial talks as on "the right track," a notable comment given Iran's recent tough rhetoric.

"It's too soon to say it has been successful," Araghchi added in an interview with Iran's Press TV. He reiterated Iran's demand that the United States lift its sanctions in one step, rather than in stages responding to Iranian moves.

State Department spokesman Ned Price also hailed what he called progress, and acknowledged the difficulty of dismantling the network of sanctions erected after Trump puled the United States out of the agreement in 2018.

"We know there will be difficult discussions ahead but again, this is a healthy step forward," Price said. He added that diplomatic contact, even at a remove, is the best way to fulfill Biden's campaign pledge to comply with the original deal so long as Iran shows it is committed to doing the same.

"The shorthand is 'compliance for compliance,' " Price said.

The talks will not resolve the core disagreement over which side, Iran or the United States, is at fault and should make the first move to repair the agreement.

While both sides still publicly maintain that the onus is on the other, in recent weeks they have moved, with help from European signatories to the deal, toward negotiating a list of sequential, simultaneous steps to return to compliance.

A U.S. team led by President Biden's special envoy for Iran, Robert Malley, was in Vienna for the discussions, which are scheduled to continue Friday. The Americans and Iranians are staying in separate hotels, and U.S. officials do not anticipate direct talks between the two delegations.

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

"We expect this to be a long process," said White House press secretary Jen Psaki. "And we continue to believe that a diplomatic path is the right path forward, and there are benefits to all sides."

For the United States, a return to the deal would provide better information about Iran's nuclear program and the threat of the country acquiring a nuclear weapon, she said.

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

For the United States and others, the hope is that diplomacy can persuade Iran to scale back its uranium enrichment to the agreed levels. Iran wants an end to sanctions initially lifted under the accord and reimposed by Trump, as well as more than 1,500 new measures Trump levied to force Iran’s leaders to make a new deal, which they never did.

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.

The talks are an important goal for Biden, who was vice president when President Barack Obama struck the original deal and who has surrounded himself with advisers who took part in the lengthy negotiations. Obama considered the Iran deal his signal foreign policy achievement, making it a top target for Trump to dismantle.

The timing and structure of the meetings represent a disappointment for those who hoped for a quick and muscular U.S. re-engagement with Iran under Biden.

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 tackling domestic priorities, including the coronavirus pandemic and its economic fallout. Any move he makes to rejoin the agreement would face stiff opposition from Republicans in Congress, potentially interfering with those domestic goals.

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 deal, Russian Ambassador to Austria Mikhail Ulyanov tweeted. Russia is among the signatories to the deal, along with China, Germany, Britain and France.

Price told reporters in Washington that one group would look at what Iran needs to do, the other at what would be needed on the U.S. side.

“I take the statements of all senior officials about aiming for a longer and stronger deal at face value,” said Suzanne Maloney, an Iran expert at the Brookings Institution, “although I recognize that the temptation to try to simply patch up the JCPOA and call it a victory is probably quite tempting.”

Maloney said the deal, formally known as the Joint Comprehensive Plan of Action, “cannot be the foremost priority for this administration at this particular moment in history” because of the imperative of addressing the pandemic.

She said that for now, the limited diplomacy may serve to calm nerves and buy time.

The overlapping layers of economic sanctions 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,” he said.

Behnam Ben Taleblu, an Iran expert at the Foundation for Defense of Democracies, said that despite their limited nature, the talks thus far are “instructive” in that the United States is not insisting on a more sweeping agreement.

“They show the U.S. as willing to dispense with broadening the deal parameters early on,” perhaps out of concern that “Iran’s politics and penchant for escalation will make the nuclear program harder to address in the future.”

Secretary of State Antony Blinken and others have said the primary goal is to put Iran’s nuclear program “in a box” that would forestall a nuclear weapon.

Patience is a virtue, Ben Taleblu said, “but this also raises concerns over premature sanctions relief or how much leverage Washington will lose in each iteration of talks.”

Morris reported from Berlin and Fahim from Istanbul.