UN atomic watchdog chief Yukiya Amano welcomed Iran's compliance with measures to lift suspicions about its nuclear programme on Monday, but cautioned that some doubt remained.
He also appealed for patience ahead of a July 20 deadline for a deal between Western powers and Iran, as the IAEA analyses data that Iran may have sought a nuclear bomb.
In a report last month, Amano revealed that Iran had so far stuck by its agreements with the IAEA and implemented all recently agreed measures, even addressing matters related to bomb-making for the first time in six years.
The so-called "possible military dimensions" (PMDs) of Iran's nuclear programme -- including the use of detonators -- have long been of concern to the international community, although Iran insists the aims of its programme are purely peaceful.
"Iran has engaged with the agency substantively, including in the clarification of issues related to the use of exploding bridge wire detonators," Amano told IAEA member states Monday.
This "has helped us to gain a better understanding of its nuclear programme," he added.
Tehran's further agreement in May to implement five new practical measures "is a further welcome step forward," he also said.
The IAEA chief warned however that IAEA analysis of information handed over by Iran -- including weapons-related information -- would take time, even as international talks with six world powers gather pace.
"I myself would like to see the speeding up of this process but these things, clarifying of issues related to PMD, require time. Rushing to assess or rushing to have everything at one time is not the right approach," Amano told journalists.
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"The issues related to PMD are like a jigsaw puzzle so it is not an effective way to rush to a conclusion."
"We need to take time and analyse carefully, and we need to have a good understanding of the whole picture."
Iran and western powers -- the so-called P5+1, including the five permanent members of the UN Security Council and Germany -- are striving to draft a potentially historic nuclear deal by July 20.
Several rounds of talks have already been held in Vienna but the latest in mid-May ended with no apparent progress on a deal.
A next round -- held separately from Iran's meetings with the IAEA -- is planned for June 16-20 in Vienna.
While welcoming Iran's cooperation with the IAEA, Amano also pointed to some lingering concern over its nuclear drive, including activities at the Parchin military base -- observed via satellite -- where Iran is suspected of conducting research for a nuclear payload.
"Since February this year, we again start to observe activities... these activities continue," Amano said Monday.
Iran has repeatedly refused the IAEA access to the facility but "we keep on insisting to have access to that particular site in Parchin, to the people and to their documents," he said.
As a result, the IAEA is "not in a position to provide credible assurance about the absence of undeclared nuclear material and activities in Iran, and therefore to conclude that all nuclear material in Iran is in peaceful activities."