The Iranian Foreign Ministry spokesman reported on Monday that Tehran and 6 world powers have made major progress in talks to revive their 2015 nuclear deal but key issues still need to be resolved.

Saeed Khatibzadeh told a televised weekly news conference that “each round of talks in Vienna could have been the final round. We should not rush. We have made significant progress but key issues remain.”

“There is no stalemate in the talks,” he added.

After former U.S. President Donald Trump abandoned the deal 3 years ago and reimposed sanctions on Tehran, Iran has been rebuilding stockpiles of enriched uranium, enriching it to higher levels of fissile purity and installing advanced centrifuges to accelerate production.

Iran’s top nuclear negotiator doubted that the talks would be the final round, and the delegations might need to return to their capitals for consultations.

Abbas Araqchi told state TV from Vienna that “the negotiations are very complex and we have now reached the main issues of dispute.”

According to sources familiar with the talks, Tehran’s reversal of its multiple breaches of the deal, and its enrichment with advanced centrifuges and production of uranium metal, are some of the remaining issues.

U.S. President Joe Biden has said his country will return to the pact if Iran first resumes compliance with its strict limits on uranium enrichment, a possible pathway to nuclear bombs.

“All sanctions should be lifted and then it should be verified by Iran … then we will reverse our nuclear steps,” Khatibzadeh said.