Iranian government spokesman Ali Rabiei announced on Tuesday that he was optimistic over his country reaching an agreement soon at talks with world powers to revive a 2015 nuclear deal, although Tehran’s top negotiator warned that serious issues remained.

Tehran and global powers have held multiple rounds of negotiations since April in Vienna, working on steps that Iran and the United States must take on sanctions and nuclear activities to return to full adherence to the nuclear pact.

Rabiei told a news conference streamed on a state-run website that “general agreements have been reached on major disputes. On the lifting of sanctions, the remaining cases are very minor, and given the negotiation process, we are optimistic about resolving the remaining minor and practical cases.”

Abbas Araqchi, Iran’s top negotiator, struck a more cautious stance in comments to state TV.

“There are still serious and important issues that need to be resolved,” Araqchi said. “Today we will start the negotiations again and we hope that during the few days of talks, God willing, we will be able to reach the final solutions.”

In turn, U.S. Special Envoy for Iran Robert Malley wrote on Twitter while on his way to the talks: “The latest round of talks was constructive and saw meaningful progress. But much work still needs to be done.”

Iranian President Hassan Rouhani said last week that sanctions on the central bank, petrochemicals, oil, shipping, and insurance had been dealt with in the talks, though European diplomats said success was not guaranteed and very difficult issues remained.

U.S. sanctions are likely to be a major issue in campaigning for Tehran’s presidential election on June 18.

On Tuesday, State TV said Iran’s election watchdog had approved the candidacy of hardline judiciary chief Ebrahim Raisi in the election. Hardliners say the United States cannot be trusted to respect any nuclear accord.