Analysts have played down the UN atomic agency's discovery of higher-grade uranium traces in Iran, saying it was likely due to a technical glitch rather than a covert attempt to enrich to arms grade.

The agency's latest report, seen by AFP Friday, did however say that satellite imagery showed "extensive activity" at the Parchin military site, which it said could hamper investigating claims of suspected nuclear weapons research there.

The International Atomic Energy Agency also revealed that its head, Yukiya Amano, wanted in a visit to Tehran on May 21 to "conclude" a deal on clarifying accusations of such research.

But Amano returned empty-handed, saying only that he and Iran's chief nuclear negotiator Saeed Jalili made a "decision" to reach an agreement, and that he expected this to be signed "quite soon."

The agency report said that the traces found at the Fordo site, inside a mountain near Qom, were of uranium enriched to purities of 27 percent.

Iran has told the IAEA that the site was enriching only to 20 percent, which was already of concern to the watchdog since the capability to do so shortens the theoretical time needed to enrich to weapons-grade uranium of 90 percent.

"Iran indicated that the production of such particles 'above the target value' may happen for technical reasons beyond the operator's control," the report said.

"The agency is assessing Iran's explanation and has requested further details. On 5 May 2012, the agency took further environmental samples from the same location.... These samples are currently being analysed," it added.

Analysts played down the discovery, with Mark Fitzpatrick from the International Institute for Strategic Studies think-tank in London saying it was "probably a technical glitch."

"There are good reasons to worry about Iran's enrichment work but this probably isn't one of them," he told AFP.

Mark Hibbs, nuclear proliferation expert at the Carnegie Foundation for International Peace, agreed, telling AFP that the discovery "isn't proof that Iran is clandestinely enriching uranium to over 20 percent."

Hibbs added however that Amano "has to be concerned about that possibility because of Iran's track record of concealment and failure to declare nuclear activities."

UN leader Ban Ki-moon said Iran has to build "international confidence" that its nuclear program is peaceful, as he welcomed the commitment to hold new talks.

The UN secretary general "hopes that Iran will take the necessary measures to build and sustain international confidence in the exclusively peaceful nature of its program," his spokesman said.

Western nations say Iran is seeking to develop a nuclear bomb. Iran says its drive is peaceful.

France called on Tehran to "cooperate unreservedly" with the UN agency "to shed light on the persistent shadowy aread of its nuclear programme," a French foreign ministry spokesman said.

Multiple UN Security Council resolutions have called on Iran to cease all enrichment activities because of the IAEA being unable to verify that they were purely for peaceful purposes.

The P5+1 powers -- the United States, Russia, China, Britain, France and Germany -- proposed in a meeting with Iran this week that Iran stop 20-percent enrichment and a suspension of all activities at Fordo, diplomats said.

Iran is however loath to do any such thing without the prospect that UN and unilateral sanctions imposed on the country in recent years -- more will hit on July 1 -- would be eased.

The P5+1's proposals stopped short of this, offering instead a series of lesser incentives that state media reports in Iran indicated Tehran thought were woefully insufficient.

The two days of intense talks in Baghdad achieved very little other than agreeing to meet again in Moscow on June 18-19.

The IAEA report also said that new satellite imagery indicated "extensive activities" were taking place at buildings at the Parchin military site near Tehran which the IAEA says it would like to inspect but Iran has denied it.

The IAEA said that "virtually no activity had been observed for a number of years" and that the apparent new work "could hamper the agency's ability to undertake effective verification."

Iran says Parchin is not a designated nuclear site and thus it is not obliged to permit IAEA inspections, although it last did so in 2005.

It says if it did allow inspections of the site, they would have to be part of an agreed "road map" that would address the IAEA's concerns in a set order.