Javier Solana, the European Union foreign policy chief and Ali Larijani, Iran's chief nuclear negotiator, met for a second day of talks on Sunday.
Although Larijani reported "some progress" after the first meeting on Saturday, the Iranian foreign ministry in Tehran said on Sunday that the question of suspending enrichment work - a key demand if sanctions are to be avoided - was a "thing of the past".
"Iran will not take a step back," ministry spokesman Hamid Reza Asefi told reporters.
Asefi said the Vienna talks between Solana and Larijani, had been "good" but that Iran "rejected any negotiations with preconditions".
The talks between Solana and Larijani have widely been seen as possibly being the last chance to avert UN Security Council moves to impose sanctions against Tehran over its atomic programme.
Larijani said on Saturday: "We have had good and constructive talks and we have made some progress in some areas."
The talks came after two days of uncertainty reflecting doubts over whether they would achieve anything.
The Solana-Larijani meeting was originally scheduled for Wednesday but postponed at the last minute.
The reluctance of both sides to commit to the talks betrayed a war of nerves that has intensified since Iran ignored a council deadline of August 31 to stop enriching uranium, a process that could yield atomic bombs.
Neither side provided details about what had been discussed at the meeting.
Before the talks, Solana had wanted Larijani to clarify Iran's
21-page reply to an offer of trade and other incentives to halt its nuclear fuel programme.
"We have had good and constructive talks and we have made some progress in some areas."
Iran's nuclear negotiator
Specifically, Solana was expected to home in on hints in the response that Tehran could curb the programme if engaged in negotiations to implement the benefits on offer.
Larijani meanwhile was expected to again rule out the precondition that enrichment be suspended indefinitely.
A diplomat from one of three EU states, France, Britain and Germany, in the sextet of powers who made the offers said: "We don't think this meeting will provide a basis for negotiations."
Regardless of the Vienna talks, Nicholas Burns, the US undersecretary of state, said after a meeting of the six in Berlin on Friday that Washington expected the council to begin deliberations next week on a draft sanctions resolution.
But key EU allies as well as Russia and China voiced growing doubt about the speed with which Washington wanted to pursue financial and diplomatic sanctions against Tehran, its arch-foe but also the world's fourth-biggest oil exporter.
To various degrees, they prefer further talks to explore a compromise that would save face on both sides.
Wen Jiabao, the Chinese premier, speaking after a China-EU summit in Helsinki on Saturday, urged the international community to exercise caution over sanctions and reiterated his government's call for Iran to "take seriously" concerns over the programme.
He said: "Sanctions or pressure will not necessarily bring about the goal of solving the Iran nuclear issue.
"Our objective is to promote an ultimate peaceful resolution of the Iran nuclear issue."