"A deal that addresses all sides' core concerns is in sight, but if it is not reached in the coming weeks, Iran's ongoing nuclear advances will make it impossible for us to return to the JCPOA," a State Department spokesperson said, referring to the 2015 framework agreement.

The talks will resume in Vienna on Tuesday after negotiators in recent weeks have cited progress in seeking to revive the 2015 accord that was supposed to prevent Iran from acquiring an atomic bomb, a goal it has always denied pursuing.

Parties have been negotiating in Vienna since last year with indirect US participation.

Talks were most recently halted at the end of last month, and the negotiators returned to their capitals for consultations.

Former US president Donald Trump unilaterally withdrew from the pact in 2018 and reimposed tough economic sanctions on Iran, prompting the Islamic republic to begin pulling back from its commitments under the deal and step up its nuclear activities.

Iranian Foreign Ministry spokesman Saeed Khatibzadeh said the answers that "the United States brings tomorrow to Vienna will determine when we can reach an agreement."

"We have made significant progress in various areas of the Vienna negotiations" including on guarantees that Iran seeks that a new US administration would not breach the deal once again, Khatibzadeh told reporters.