Tehran has been engaged in long-running negotiations in the Austrian capital 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.

"We will not be going to Vienna for new negotiations but to finalise the nuclear agreement," foreign ministry spokesman Saeed 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."

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

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

- 'Final phase' -

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.

Iranian and US delegations in Vienna do not communicate directly but 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 Islamic Revolutionary Guard Corps, the ideological arm of Iran's military.

Washington recently confirmed that sanctions on the Guards would stay.

On Wednesday, the US Treasury announced measures targeting several entities it accused of involvement in procuring supplies for Iran's ballistic missile programme.

A day later, Khatibzadeh said Washington's imposition of the fresh sanctions on the Islamic republic showed its "ill will" towards Iran.

On Monday, Khatibzadeh levelled further criticism at the US.

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

On Sunday, however, Iran's Foreign Minister Hossein Amir-Abdollahian said an agreement was "close", during a phone conversation with UN Secretary-General Antonio Guterres.

"We have passed on our proposals on the remaining issues to the American side through the EU senior negotiator, and now the ball is in US court," Iran's top diplomat said.