Iran and the UN atomic watchdog IAEA were tight-lipped on Monday after talks aimed at improving transparency and clearing up long-standing allegations of past efforts by Tehran to develop nuclear weapons.
International Atomic Energy Agency spokeswoman Gill Tudor said only that the meeting in Vienna "has ended" and that the United Nations agency would make no further comment.
Iran's delegation, led by Iranian ambassador to the IAEA Reza Najafi, left the talks without speaking to reporters.
The meeting comes ahead of the next round of parallel but linked talks this week, also in Vienna, between Iran and the five permanent members of the UN Security Council plus Germany.
The IAEA has long been seeking answers from Iran over what it calls "overall, credible" evidence that before 2003, and possibly since, Tehran has conducted research into making nuclear weapons.
The Islamic republic has rejected such claims, saying they are based on faulty intelligence from the CIA and Israel's Mossad -- intelligence that it has not been allowed to see.
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Some progress was made last year when Iran promised to clarify its use of Exploding Bridge Wire (EBW) detonators, devices which could theoretically be used in an atomic bomb but which also have a range of other uses.
The deadline to provide answers on this, and on six other steps to improve transparency such as providing more design information on Iran's new Arak reactor under construction, is on Thursday.
Experts say other, much trickier questions remain to be resolved.
Other claims outlined in a major November 2011 IAEA report include alleged explosives testing at the Parchin military base near Tehran that the watchdog says would be "strong indicators" of missile development.
Iran and the six world powers aim to turn into a lasting accord a temporary deal from November under which Iran scaled back certain nuclear activities for six months in return for minor relief from painful UN and Western sanctions.
As part of this sought-after comprehensive accord, as well as reducing in scope its nuclear activities, the six powers want Iran to answer all the IAEA's outstanding questions.
Iranian Foreign Minister Mohammad Javad Zarif is due to hold a working dinner with the powers' main negotiator, EU foreign policy chief Catherine Ashton, on Tuesday before the talks start in earnest on Wednesday.