TEHRAN - Indirect talks between Tehran and Washington on reviving the 2015 Iran nuclear deal have come closer than ever to an agreement, but essential issues remain to be negotiated, the top Iranian negotiator said on Thursday.

Iran and six world powers have been negotiating in Vienna since April to work out steps for both sides to take. The United States withdrew in 2018 from the pact, under which Iran accepted limits on its nuclear program in exchange for a termination of economic and financial sanctions.

"We achieved good, tangible progress on the different issues .... we are closer than ever to an agreement but there are still essential issues under negotiations," Iranian Deputy Foreign Minister Abbas Araqchi was quoted as telling Al Jazeera television.

Araqchi said Iran's presidential election, which is being held on Friday, would have no effect on the negotiations and the Iranian negotiating team will continue the talks regardless of domestic policy.

Iran’s new president is expected to name his Cabinet by mid-August. Current President Hassan Rouhani’s term ends on Aug. 3.

The sixth round of talks resumed on Saturday with the remaining parties to the deal - Iran, Russia, China, France, Britain, Germany and the European Union - meeting in the basement of a luxury hotel.

The U.S. delegation to the talks is based in a hotel across the street as Iran refuses face-to-face meetings. Iran has said it will not negotiate directly until the U.S. rejoins the nuclear deal and lift the illegal sanctions against Iran in a verifiable manner.

One year after former U.S. President Donald Trump pulled out of the deal and reimposed sanctions on Iran, Tehran announced that its “strategic patience” is over and embarked on counter measures, including rebuilding stockpiles of enriched uranium. Trump quit the nuclear deal, officially known as the Joint Comprehensive Plan of Action (JCPOA), despite the fact that the Iran was fully loyal to the terms of the agreement.

"We want to make sure that what happened when Trump pulled out of the deal will not be repeated by any other American president in the future," Araqchi told the pan-Arab satellite TV network.

Iranian Foreign Ministry spokesman Saeed Khatibzadeh has also echoed similar remarks, saying Tehran wants to make sure that a mechanism should be devised that the U.S. would not withdraw from the agreement in the future.

The EU has expressed optimism about the Vienna talks. 

"We are making progress, but the negotiations are intense and a number of issues [remain], including on how steps are to be implemented," EU representative Alain Matton told reporters in Vienna.

"The EU will continue with the talks with all the participants, and separately with the U.S., to get very close to a final agreement in the coming days," Matton added. 

On Thursday, Russia's envoy to the talks, Mikhail Ulyanov, added a note of caution, saying progress had been made in the last few days but talks were tough.

"Some difficult and time-consuming topics still remain unresolved," Ulyanov said.

“The #ViennaTalks are getting progress. Over the last couple of days we achieved more clarity with regard to one of the most controversial issues-Implementation plan (who should do what and when). However some difficult and time-consuming topics still remain unresolved,” the Russian diplomat tweeted.

The French Foreign Ministry said on Wednesday there were still significant disagreements.

In Washington, State Department spokesman Ned Price repeated the U.S. view that the Vienna talks had made progress since they began in April but that challenges remained, saying he could not put a "time frame" on when the current round might end.

He declined to say whether Araqchi's assessment or Ulyanov's was more accurate. "We have made progress between rounds one and six but ... I don’t want to be definitive in embracing one assessment over another," he said, according to Reuters.

On June 12, Germany urged quick progress during this latest round of negotiations.

"It is about flexibility and pragmatism from all participating parties," German Foreign Minister Heiko Maas told Reuters.

"Playing for time is in no one's interest," he added.

On Wednesday, Mahmoud Vaezi, the Iranian president’s chief of staff, expressed hope that the nuclear talks underway in Vienna would result in a piece of good news but underlined that there would be no agreement until all Iranian demands are met.

Speaking on the sidelines of a cabinet meeting, Vaezi also underlining that the presidential elections in Iran should not cause any “harassment” for the talks.

“In most countries, foreign policy is separate from domestic and factional issues. Therefore, given the importance of the Vienna talks, the principles we adhere to are the framework outlined by the Leader of the Islamic Revolution,” Vaezi said, according to Iran’s state news agency IRNA.

 “We are conducting these negotiations within the framework drawn by the Leader of the Islamic Revolution and approved by the establishment. So, the negotiating team is moving in that direction.”

Vaezi added, “What has been agreed so far is most important economic issues that remain as an understanding, but the negotiating team has said that whatever we have agreed on will be ‘final’ when we meet all our demands. For this reason, the rest of the cases are being discussed. We hope to have good news on this in the coming weeks.”

The European Union and the United States have said that lifting sanctions on Iran constitutes an “essential part” of the nuclear agreement.

“The European Union and the United States recognize that, alongside the return to full and effective implementation by Iran of its nuclear-related commitments, the lifting of sanctions constitutes an essential part of the JCPOA,” the EU and the U.S. said in a joint statement after a summit on Tuesday.

Trump’s successor, Joe Biden, has promised to rejoin the JCPOA while strongly criticizing the former hawkish U.S. administration’s Iran policy, including its failed “maximum pressure” campaign.

Mark Fitzpatrick, an associate fellow at the International Institute for Strategic Studies (IISS), says that "strategy of continuing maximum pressure has been proven to be ineffective."

"I frankly do not pay much attention anymore to opponents of the JCPOA, who no longer are in power. They should be ignored because their preferred strategy of continuing maximum pressure has been proven to be ineffective," Fitzpatrick tells the Tehran Times.