Iran gave an upbeat assessment on Tuesday of two days of talks with the top investigator of the U.N. atomic energy watchdog, who was looking into Western reports that Iran secretly studied how to design nuclear bombs.
"The talks with (Olli) Heinonen were positive," a senior Iranian nuclear official, who asked not to be named, told Reuters . He did not say what was discussed.
International Atomic Energy Agency (IAEA) officials said it would have no comment before Heinonen returned to Vienna on Wednesday.
Diplomats close to the IAEA had said the point of the talks was to get substantive Iranian responses to intelligence reports alleging illicit nuclear bomb research. Iran has denied the reports but not given evidence to support its stance, they say.
Iranian officials had said Heinonen's visit was intended to "advance cooperation" with the IAEA, the U.N. body investigating Iran's nuclear ambitions, and that Tehran had already settled the intelligence issue in previous exchanges.
Iran says its nuclear program is a peaceful drive to produce electricity so that the world's fourth-largest crude exporter can sell more of its oil and gas abroad.
However, it has failed to convince the West, which believes Tehran is seeking technology so it can build atomic weapons.
U.S. President George W. Bush and British Prime Minister Gordon Brown last week vowed to prevent Iran from developing a nuclear bomb, possibly by expanding sanctions.
German Chancellor Angela Merkel expressed frustration on Tuesday about the lack of headway in the dispute with Iran.
"We would welcome it a lot if we could make progress here," Merkel said at a joint news conference in Berlin with Qatar Prime Minister Sheikh Hamad bin Jassim bin Jabr al-Thani. "Unfortunately, we're not really moving ahead on this."
For his part, Sheikh Hamad urged dialogue rather than punitive measures: "Sanctions are counterproductive," he said. "To us in the region, it is very important that the problem is solved peacefully and not with violence."
The U.N. Security Council has imposed three sets of limited sanctions on Iran since 2006 because of its inadequate cooperation with IAEA investigations.
The five permanent U.N. Security Council members - Britain, China, France, Russia and the United States -- plus Germany have been looking to revive nuclear negotiations with Iran.
Russian Foreign Minister Sergei Lavrov said he wanted the next round of six-nation talks to be held in London, at the same time as a meeting of the quartet of Middle East peace mediators on May 1-2.
The IAEA's Heinonen raised a diplomatic stir in February with a presentation that indicated links in Iran between projects to process uranium, test explosives and modify a missile cone in a way suitable for a nuclear warhead.
Iran has dismissed the intelligence as baseless, forged or irrelevant. The IAEA says the information remains unverified but warrants thorough investigation to enable it to wind up a long inquiry into Iran's secretive quest for nuclear power.
IAEA Director Mohamed ElBaradei has said the world "needs to make sure Iran did not have a weapons program".