DUBAI (Reuters) -Iran’s chief negotiator said on Tuesday talks to save the 2015 nuclear accord were moving forward despite difficulties but warned Tehran would stop the negotiations if faced with “unreasonable demands” or time wasting.

Iran and world powers have made headway in the Vienna talks though much more work is needed, a senior European Union official said, with meetings to resume next week after consultations in their respective capitals.

Iranian Deputy Foreign Minister Abbas Araqchi “assessed the current trend of the talks as going forward, despite the existing difficulties and challenges,” Iranian state media reported.

“The Iranian delegation will stop the talks whenever the process of negotiations leads to unreasonable demands, waste of time and irrational bargaining,” Araqchi was quoted as saying.

“It is too early to judge the outcome or to say whether we are optimistic or pessimistic, but we think we are on the right track,” Araqchi told state television.

Hardline-led Iranian news agencies quoted an unnamed source as saying the United States was only planning to issue temporary waivers instead of permanently lifting sanctions, which Washington re-imposed on Tehran after withdrawing from the nuclear accord in 2018.

“America’s intention is not to lift the sanctions completely and to be satisfied with temporary waivers on some sanctions in order to simply return to the nuclear accord so that it can use the possibility of the snapback mechanism against Iran,” Fars news agency quoted the source as telling state-run Press TV.

But Araqchi, widely seen as a pragmatist, said on Twitter: “I don’t know who the ‘informed source’ of Press TV in Vienna is, but s/he is certainly not ‘informed’.”

Under the 2015 deal, Iran agreed to curb its nuclear work in return for relief from U.S. and other sanctions. The accord includes the option of a snapback of U.N. sanctions if Iran breaches the deal, requiring Tehran to suspend all nuclear enrichment-related activities, including research development.

Earlier on Tuesday, Iran’s government said it began enriching uranium to 60% purity to show its technical capacity after a sabotage attack at a nuclear plant, and added the move is quickly reversible if the United States lifts sanctions.

(Reporting by Dubai newsroom;Editing by Alison Williams and Cynthia Osterman)