Iran’s and Europe’s top negotiators have returned home for discussions as efforts to resurrect the 2015 nuclear agreement reach a critical juncture, according to official media in the Islamic republic on Saturday.

“The negotiators will return to Vienna in two days,” according to IRNA, although expert-level discussions at the eighth round of negotiations will continue on Saturday and Sunday.

The discussions between Tehran and the international powers began in late November after being stopped for around five months due to Iran’s election of a new, ultraconservative administration.

Iran reached an agreement on its nuclear program, known formally as the Joint Comprehensive Plan of Action (JCPOA), with the United Kingdom, China, France, Germany, Russia, and the United States.

It promised sanctions relief to the Islamic Republic in exchange for restrictions on its nuclear development.

However, in 2018, former US President Donald Trump unilaterally withdrew from the accord and imposed severe penalties, causing Iran to begin withdrawing from its obligations.

“We are currently at a point in the discussions where we are discussing tough topics and how we can transform the themes that we agreed on in principle into words and insert them into a document,” an unidentified source told IRNA.

“We’re talking about the technicalities,” the source said, adding that “this is one of the most tedious, arduous, and tough phases of the discussions, but it’s absolutely necessary for us to achieve our goal.”

Tehran wants verification of the sanctions relief, as well as assurances that Washington will not back out of the deal again.

“With regard to the three areas (sanctions relief, nuclear obligations and execution, sequencing and verification), there are still unresolved problems, some of which are difficult,” the person added.

The negotiators’ return to their respective capitals comes as EU foreign policy head Josep Borrell stated on Friday that a deal with Iran was still “possible,” and that discussions were progressing in a “better environment” than before Christmas.

On Monday, Iran’s foreign ministry spokesperson, Saeed Khatibzadeh, stated that efforts by “all parties” to resurrect the nuclear accord had resulted in “excellent progress.”

French Foreign Minister Jean-Yves Le Drian recently stated that progress in the negotiations has been made, but that it has been “very sluggish.”

According to US Secretary of State Antony Blinken, there are just “a few weeks” left to rescue the 2015 agreement, and Washington would examine “alternative options” if the discussions fail.