After months of on-off negotiations, EU foreign policy chief Josep Borrell added to hopes of a renewed deal.

"I strongly believe an agreement is in sight," he said on Twitter, after a call with Iran's foreign minister.

"The moment has come to make an ultimate effort and reach a compromise," Borrell said, in his dual capacity as coordinator of the Joint Comprehensive Plan of Action (JCPOA), the deal agreed between Iran and world powers in 2015.

But Iranian President Ebrahim Raisi, receiving Irish foreign and defence minister Simon Coveney on Monday, said that "sanctions must be truly lifted" in the ongoing negotiations to revive the deal in the Austrian capital.

"The rights of the Iranian people must be respected," he added.

The talks involve Iran, Britain, China, France, Germany and Russia directly, and the United States indirectly.

The 2015 deal offered Tehran sanctions relief in exchange for curbs on its nuclear programme, but the US unilaterally withdrew in 2018 under then-president Donald Trump and reimposed stiff economic sanctions, prompting Tehran to begin rolling back on its commitments.

US State Department spokesman Ned Price struck a cautious note late Monday.

"We are going to be a bit more circumspect in terms of progress that we may be seeing on the ground in Vienna," he told reporters. "Time is almost out".

- 'Serious and committed' -

Raisi hailed Ireland's "independence from the United States and some European countries", adding that the two countries have "vast capabilities" to strengthen ties, according to state news agency IRNA.

Ireland occupies one of the non-permanent seats on the UN Security Council this year, and did so last year. It has been acting as facilitator for Resolution 2231, which was agreed in 2015 to endorse the JCPOA.

Also on Monday, Iran's Foreign Minister Hossein Amir-Abdollahian said that he believed a revived agreement is "at hand", provided the US and European parties "are serious" about returning to full compliance with the deal.

Coveney said he believed the Western parties to the 2015 deal are "deeply committed to making this work," addressing reporters alongside his Iranian counterpart.

"I discussed with Iranian counterparts the steps now needed to finalise a return to an agreement that will provide significant benefits to the Iranian economy, through the lifting of sanctions," Coveney added.

- 'Complicated and difficult' -

Negotiations in Vienna 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.

"Our goal is the full removal of all JCPOA sanctions," Amir-Abdollahian said, adding that "it is better for the Islamic republic to reach an agreement today rather than tomorrow".

The Iranian top diplomat's remarks followed comments by his ministry's spokesman Saeed Khatibzadeh, who said earlier on Monday that "there is no impasse in Vienna".

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

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

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

The drive to salvage the 2015 nuclear deal resumed in late November after a pause following Raisi's election in June.