"Verification and providing a guarantee is an integral part of a good deal," the secretary of Iran's Supreme National Security Council, Ali Shamkhani, wrote in a Twitter post.

"Proven American malpractice is the most important threat to any agreement," he said, adding that "the real lifting of sanctions means that #Iran will enjoy credible and sustainable economic benefits".

The accord 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 heavy economic sanctions, prompting Iran to begin rolling back on its commitments.

The Vienna talks, which involve Iran as well as Britain, China, France, Germany and Russia directly and the United States indirectly, resumed in late November after a pause following the election of ultraconservative President Ebrahim Raisi in June.

Irish Foreign and Defence Minister Simon Coveney, whose country acts as facilitator for UN Resolution 2231 endorsing the 2015 deal, visited Tehran on Monday.

Coveney said he believed the Western parties to the accord are "deeply committed to making this work," while addressing reporters alongside Iranian Foreign Minister Hossein Amir-Abdollhain.

The Iranian top diplomat said he believed an agreement was "at hand", provided the US and European parties "are serious" about returning to full compliance with the deal.

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.

Raisi, who also discussed the progress of the talks with Coveney, said "sanctions must be truly lifted" and "the rights of the Iranian people must be respected" in any agreement.

Also on Monday, EU foreign policy Chief Josep Borrell tweeted that "I strongly believe an agreement is in sight," following a call with Iran's foreign minister.