Iran was preparing to hold indirect talks with the United States in Vienna Tuesday in a bid to make Washington return to the 2015 nuclear deal and lift crippling sanctions.

Friday's announcement that Washington and Tehran would begin indirect talks through intermediaries was one of the first signs of tangible progress in efforts to return both nations to the terms of the accord, which bound Iran to restrictions in return for relief from the U.S. and international sanctions.

Then-President Donald Trump pulled the U.S. out of the accord in 2018, opting for what he called a maximum-pressure campaign of stepped-up U.S. sanctions. Since then, Iran has been steadily violating the restrictions of the deal, such as the amount of enriched uranium it can stockpile and the purity to which it can enrich it.

Some argue that Tehran’s moves have been calculated to put pressure on the other nations in the deal – Russia, China, France, Germany and Britain – to do more to offset crippling sanctions reimposed under Trump.

U.S. President Joe Biden came into office saying that getting back into the accord and getting Iran's nuclear program back under international restrictions was a priority. But Iran and the U.S. have disagreed over Iran's demands that sanctions be lifted first.

Senior foreign ministry officials from the countries still in the accord, the Joint Comprehensive Plan of Action (JCPOA), are holding a European Union-chaired meeting Tuesday in Vienna.

Also due in the Austrian capital is a U.S. delegation headed by the administration's special envoy for Iran, Rob Malley. State Department spokesperson Ned Price said talks will be structured around working groups that the Europeans will form with the other parties to the accord.

Price said Monday the talks are a "healthy step forward” but added that "we don’t anticipate an early or immediate breakthrough as these discussions, we fully expect, will be difficult.”

"We don’t anticipate at present that there will be direct talks with Iran," he said, according to remarks carried by The Associated Press (AP). "Though of course, we remain open to them. And so we’ll have to see how things go starting early this week.”

Iranian Foreign Minister Mohammad Javad Zarif wrote on Twitter Friday, "No Iran-US meeting. Unnecessary.”

A statement issued after parties of the accord met virtually Friday said the aim of their meeting Tuesday is to "clearly identify sanctions lifting and nuclear implementation measures, including through convening meetings of the relevant expert groups.”

Adding fresh doubt to chances of a breakthrough, an Iranian official told Reuters: "Our agenda during the meeting will be the removal of all U.S. 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 final say on all state matters, has opposed any gradual easing of sanctions.

Ahead of Tuesday's talks, an Iranian prosecutor said 10 officials have been indicted over last year's military shooting-down of a Ukrainian passenger plane in which 176 people died. Iran faced withering international criticism last month for releasing a final report that blamed human error but named no one responsible.