The U.S. and Iran are holding indirect talks on Tuesday in what could be the first step toward reviving the 2015 deal that constrains Tehran’s nuclear facilities in exchange for sanctions relief.

The meetings, hosted by the European Union, will work toward two separate agreements — on how the U.S. and Iran can both return to compliance with the deal’s terms.

Former President Donald Trump withdrew from the accord in 2018, re-imposing strict U.S. sanctions on Iran’s economy and government. In response, the Iranian government began violating the restrictions on its nuclear program, now enriching more uranium, at higher levels, and with more — and more advanced — centrifuges.

While both sides have downplayed Tuesday’s meetings, it could mark the beginning of a quick return to the deal, which saw Iran agree to international inspections of and certain time-limited restrictions on its nuclear program. In exchange, sanctions were lifted by the United Nations, the U.S., and the other world powers who signed on — the United Kingdom, France, Germany, Russia and China.

But especially amid fierce pressure in Washington and from regional allies like Israel, a renewed nuclear deal would still leave a long road ahead for the Biden administration, which seeks to “lengthen and strengthen” the original agreement, including by addressing Iran’s support for proxy forces across the region and its ballistic missile program.

“We don’t underestimate the scale of the challenges ahead. These are early days. We don’t anticipate an early or immediate breakthrough,” State Department spokesperson Ned Price said Monday — before calling the indirect talks a “healthy step forward.”

U.S. special envoy for Iran Rob Malley will lead the delegation, which will meet with European, Chinese and Russian counterparts. Iran’s delegation will have its own separate meetings with those teams, which together constitute the remaining members of the agreement.

While there is no face-to-face meeting planned with Iran, the U.S. still “remains open” to one, according to Price. The U.S. first offered to meet the Iranians in February, but weeks of outreach through European allies have been rejected by the Iranian government.

Just last week, Iranian Foreign Minister Javad Zarif tweeted a face-to-face meeting was “unnecessary.” Iran has called for the U.S. to move first in returning to the deal since Trump was the first to violate it, even refusing to meet the Americans until the U.S. government begins lifting sanctions — while Biden has vowed to keep sanctions in place until Iran returns to compliance.

“What is promising about the Vienna talks is there seems to be an understanding now by both the Biden administration and the Iranians that neither side is going to go first. It may seem like semantics, and maybe it is. But I think what we have is at least the germ of a process whereby both sides can say we’re moving simultaneously, both sides can save a little face and put together a roadmap going forward,” said Suzanne DiMaggio, a senior fellow at the Carnegie Endowment for International Peace, a Washington-based think tank.

The talks begin Tuesday in Vienna, the capital of Austria, where much of the diplomacy around the original deal unfolded. While they’re slated for one day, it could extend on, especially if the separate “working groups” make progress and bring in technical experts to finalize details on Iranian and U.S. compliance.

That alone will be no easy task. There are critical questions remaining, including how quickly Iran can dismantle the advanced centrifuges now spinning, how it will reduce its stockpile of enriched uranium, and what to do about its gains in research and development as it violated its terms of the deal.

On the U.S. side, there are questions around which sanctions the Biden administration will lift, after the Trump administration targeted hundreds of Iranian businesses, officials, economic sectors, and state-owned firms and agencies.

Iran has called for all Trump-era sanctions to be removed, saying they violated the deal. But the State Department indicated Monday that it will only lift those related to Iran’s nuclear program.

“We certainly will not entertain unilateral gestures or concessions to get Iran — to induce Iran to a better place,” Price said during the department’s daily press briefing. “We’ll continue to be guided by what the original JCPOA called for, which is nuclear sanctions.”

But Press TV, an Iranian state media outlet, reported that Malley and the U.S. delegation will “leave Vienna empty-handed if the Tuesday meeting would result in anything other than the removal of all U.S. sanctions,” citing an “an informed source close to the Iranian negotiating team.”