Iran will only return to Vienna to finalise an agreement to revive its landmark 2015 nuclear deal with world powers, foreign ministry spokesman Saeed Khatibzadeh said Monday.

"We will not be going to Vienna for new negotiations but to finalise the nuclear agreement," Khatibzadeh told reporters in Tehran.

However, Iran said there were still outstanding issues that it was waiting on Washington to settle.

"At the moment, we do not yet have a definitive answer from Washington," Khatibzadeh said.

"If Washington answers the outstanding questions, we can go to Vienna as soon as possible."

Tehran has been engaged in negotiations to revive the deal, known formally as the Joint Comprehensive Plan of Action (JCPOA), with Britain, China, France, Germany and Russia directly, and the United States indirectly.

Iranian and US delegations in Vienna do not communicate directly, but messages are passed through other participants and the European Union, the talks' coordinator.

Nearly a year of negotiations have brought the parties close to renewing the 2015 accord.

But the talks were halted last month, after Russia demanded guarantees that Western sanctions imposed following its invasion of Ukraine would not damage its trade with Iran.

Russian Foreign Minister Sergei Lavrov later said Moscow had received the necessary guarantees from Washington on trade with Iran.

Among the key sticking points is Tehran's demand to remove from the US terror list the Revolutionary Guards, the ideological arm of Iran's military.

Washington recently confirmed that sanctions on the Guards would stay.

"Today, in the final phase, the United States seeks to deprive Iran of the economic benefits of the agreement," Khatibzadeh said.

The JCPOA gave Iran sanctions relief in exchange for curbs on its nuclear programme to guarantee that Tehran could not develop or acquire a nuclear weapon -- something it has always denied wanting to do.

But the US unilateral withdrawal from the accord in 2018 under then-president Donald Trump and the reimposition of biting economic sanctions prompted Iran to begin rolling back its own commitments.

The Vienna talks aim to return the US to the nuclear deal, including through the lifting of sanctions on Iran, and to ensure Tehran's full compliance with its commitments.