Iran's chief negotiator Ali Larijani, who agreed a "plan of action" for transparency with Mohamed El Baradei, the IAEA director, in talks on Friday, returned to see El Baradei on Sunday and issued the invitation, the IAEA said in a statement.
Olli Heinonen, the IAEA's global head of inspectors, was expected to lead the team after his return from North Korea, where he will begin a five-day visit on Tuesday to lay groundwork for Pyongyang's promised nuclear disarmament.
"Iran has a right to pursue nuclear power but it should abide by international agreements and laws"
New York, US
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Iran says that its nuclear programme is only for electricity. But Western countries suspect that Tehran wants to build a nuclear bomb as it hid sensitive atomic research from the IAEA until 2003 and has stonewalled investigations since then.
Tehran has been hit with two sets of mild UN sanctions for defying calls to stop enrichment and open up to IAEA inquiries.
Washington voiced scepticism about the deal.
"Iran's track record is not particularly noteworthy or particularly likely to give me or anyone else confidence that anything will come of these discussions," Tom Casey, deputy spokesman for the US State Department, told reporters.
Cooperation with IAEA
A year ago, Iran pledged to come up with a plan for full cooperation with IAEA officials but never followed through.
"This [new deal] is the first break in a stalemate that has been going on since then on allowing the IAEA to resolve these remaining mysteries," said a diplomat close to the IAEA.
The Larijani-El Baradei deal calls for the IAEA and Tehran to agree within two months what steps will be needed to "let the IAEA get to the bottom of the issues", the diplomat said.
This would likely entail access for IAEA inspectors to Iranian sites, documents and officials at the centre of indications of past covert activity with military links.
Iran has long made full transparency conditional on the UN Security Council first returning authority over its file to the IAEA.
Western powers reject such a concession, saying dropping sanctions options would relieve pressure on Iran to cooperate.
The diplomat said Larijani did not repeat that demand in his meetings
with El Baradei. But, asked whether wider, stiffer sanctions now being contemplated could unravel the IAEA-Iran deal, the diplomat said: "Surely".
"I assume Iran feels pressure mounting, that there will be another Security Council resolution and they'd like to head that off, buy more time"
A Western diplomat in Vienna
A Western diplomat in Vienna said Iran's effort to address the lack of international trust in its nuclear agenda looked more like a delaying tactic than a genuine turnaround.
"I assume Iran feels pressure mounting, that there will be another Security Council resolution and they'd like to head that off, buy more time," said the diplomat.
"If Iran were really serious, why do they need to negotiate two months on how to resolve the issues? The IAEA has made clear for some time what Iran needs to do. I don't think unity (among world powers) will fall apart over this Iranian move," he said.
But Russia, one of five powers with a Security Council veto, has hinted at disagreement with a US thrust for new sanctions by saying it will back them only once the IAEA has exhausted all possibilities to resolve questions about Iran's programme.