"The negotiations are complicated and difficult as they have reached key issues that need serious political decisions especially by Washington," foreign ministry spokesman Saeed Khatibzadeh told a news conference.

But "there is no impasse in Vienna", he added.

Iran is engaged in negotiations with Britain, China, France, Germany and Russia directly and with the United States indirectly to revive the deal formally called the Joint Comprehensive Plan of Action (JCPOA).

The deal offered Tehran sanctions relief in exchange for curbs on its nuclear programme.

But the US unilaterally withdrew from the JCPOA in 2018 under then-president Donald Trump and reimposed stiff economic sanctions, prompting Tehran to begin rolling back on its commitments.

The drive to salvage the 2015 nuclear deal resumed in late November.

"If the US and European parties show real determination", an agreement can be reached very soon in Vienna, Khatibzadeh said on Monday.

"We need objective guarantees to make sure the US does not leave the agreement once again and that it honours its commitments," he added.

The goals of the negotiations are to have the US return to the deal by lifting its anti-Iran sanctions and to have Iran return to full compliance with its commitments under the agreement.

"All JCPOA sanctions with any labels must be lifted at the same time," Khatibzadeh said.

Different parties to the agreement have in recent weeks signalled progress in the talks with some saying the negotiations are in their final stage.

Ali Shamkhani, secretary of Iran's Supreme National Security Council, in a tweet on Monday said the negotiations have reached the stage the can be commented on with "certainty" and not speculation.

"The US political decision to realise or refuse to accept the requirements of forming a reliable and lasting agreement based on the principles accepted in JCPOA can replace speculations," he said.