DUBAI June 17 (Reuters) - 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 curbs on its nuclear programme in exchange for a lifting of many foreign sanctions against it. "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 on Friday would have no effect on the negotiations and the Iranian negotiating team will continue the talks regardless of domestic policy. 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. read more The U.S. delegation to the talks is based in a hotel across the street as Iran refuses face-to-face meetings. Since former U.S. President Donald Trump pulled out of the deal and reimposed sanctions on Iran, Tehran has embarked on counter measures, including rebuilding stockpiles of enriched uranium, a potential pathway to nuclear bombs. "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. 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," he said. France"s 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. Iran"s new president is expected to name his Cabinet by mid-August. Current President Hassan Rouhani"s term ends on Aug. 3.