“The American side says enrichment must stop in Iran, and if that is their goal, there will be no agreement,” Abbas Araghchi said in an interview with state media.
“We will not give up our rights, and our nuclear program, including enrichment, must continue."
Iran insists its disputed nuclear program is peaceful and sees domestic enrichment as a right enshrined by international covenants.
Western countries and Mideast arch-nemesis Israel doubt Iran's intentions.
US state department spokesperson Tammy Bruce said on Thursday that Washington was optimistic about the talks which are due for a fifth round in Rome on Friday because Iranian diplomats were still talking despite the hard US line on enrichment.
"The fifth round of the nuclear talks would not be happening if we didn't think that there was potential for it," Bruce told reporters on Thursday.
"Clearly we believe that we are going to succeed," she added. "Most of our leaders have who have spoken on it (have said talks are) about no enrichment, and the Iranians are (still) at that table. So they also understand what our position is, and they continue to go."