Mohamed ElBaradei said the talks will continue on Tuesday
Talks between Iran and world powers on a uranium enrichment deal are "off to a good start", the head of the United Nations' nuclear watchdog has said.
Russia, France and the US were at the IAEA meeting in Vienna.
Under a proposed deal, Iran would ship enriched uranium abroad to be converted for use in a research reactor.
Tehran sent a lower-level delegation led by its IAEA envoy, not its atomic agency chief, indicating a final agreement may not be reached this week.
Despite the setback, Mohamed ElBaradei, who heads the International Atomic Energy Agency (IAEA), sounded positive.
"We had this afternoon quite a constructive meeting. We're off to a good start. Most of the technical issues have been discussed," he said after the meeting at the IAEA headquarters.
He said talks would resume on Tuesday morning.
The head of the Iranian delegation, Ali Asghar Soltanieh, said he endorsed Mr ElBaradei's comments.
He refused to comment on Iranian media reports claiming that Tehran may be reluctant to ship out its fuel.
Before talks began, state broadcaster Press TV said Iran wanted to import highly enriched uranium for its research reactor by buying it directly from France, Russia or the US.
Also before the meeting, Iran's nuclear energy agency spokesman, Ali Shirzadian, said the deal would not mean the suspension of Tehran's enrichment activities.
He added that if the talks in Vienna failed, Iran would also enrich uranium to higher levels than it is currently doing.
The BBC Tehran correspondent, Jon Leyne, reports from London that the Iranian media reports, if true, would be a major blow to the West's hopes of a new dialogue.
Western powers say Iran is seeking a nuclear weapon, a charge Iran denies. It says its nuclear programme is for purely civilian, peaceful purposes.
No new sanctions?
The proposed deal hinges on an arrangement in principle that Western negotiators announced after talks in Geneva earlier this month.
Under the planned agreement, Russia and France would treat low-enriched Iranian uranium and turn it into fuel rods for a research reactor in Tehran.
Our correspondent says in theory this is a deal that could suit everyone.
Iran would have the fuel it needs, tacit acknowledgement of its right to enrich uranium, and no new sanctions.
The West would get a guarantee that Iran's existing stockpile will not be diverted to make nuclear bombs.
But the talks in Vienna have many details to resolve, our correspondent adds.
Last month, the revelation of a second uranium enrichment plant in Iran further raised Western fears that Iran was trying to develop nuclear weapons.
The Iranian government has said it will allow IAEA inspectors into the site, thought to be near the holy city of Qom.