Iran, the United States and the European Union said Wednesday they would send senior representatives to Vienna amid what appears to be a last-ditch effort at reviving talks over Tehran’s tattered 2015 nuclear deal with world powers.

It wasn’t immediately clear if all other parties to the landmark accord would attend the surprise summit, nor if there had been any progress after a monthslong stalemate and recent fruitless round of indirect talks between Iran and the U.S. in Doha.

The European Union official who chairs the talks, Enrique Mora, said the negotiations would focus on the most recent draft to restore the agreement, while Iran’s chief nuclear negotiator, Ali Bagheri Kani, said he was heading to the Austrian capital “to advance the negotiations.”

U.S. Special Representative for Iran Rob Malley wrote on Twitter that he was preparing to travel to Vienna for talks. He cautioned that American “expectations are in check” ahead of the negotiations.

“The United States welcomes EU efforts and is prepared for a good faith attempt to reach a deal. It will shortly be clear if Iran is prepared for the same,” Malley added.

The German Foreign Ministry said Berlin would be represented at an “expert level” at the talks in Vienna, adding it supported efforts to fully revive the deal “even if hopes are very small.” It again pushed Iran to conclude the deal and said that would mean “giving up maximalist positions in areas beyond” the nuclear agreement.

Russia’s chief representative at the talks, Mikhail Ulyanov, also wrote on Twitter that negotiators from Russia, a key signatory of the nuclear deal, “stand ready for constructive talks in order to finalize the agreement.”

The prospects for the deal’s restoration have darkened in the past few months with major sticking points remaining, including Tehran’s demand that Washington provide guarantees that it won’t again quit the pact and that it lift terrorism sanctions on Iran’s paramilitary Revolutionary Guard.

The abruptly called meeting in Vienna comes after EU foreign affairs chief Josep Borrell has repeatedly pushed to break the deadlock and salvage the deal in past weeks. He recently wrote in The Financial Times that “the space for additional significant compromises has been exhausted.”

Former President Donald Trump withdrew the U.S. from the 2015 deal, which lifted most international sanctions on Tehran in exchange for tight restrictions on Iran’s nuclear program. Since then, Iran has massively expanded its nuclear work and now has enough highly enriched uranium to fuel one nuclear weapon, according to nonproliferation experts.

However, Iran still would need to design a bomb and a delivery system for it, likely a monthslong project. Iran insists its program is for peaceful purposes, though U.N. experts and Western intelligence agencies say Iran had an organized military nuclear program through 2003.

(AP)