Iran's track record of hiding nuclear activities means the UN cannot be sure about what Iran is doing now, the head of the UN's nuclear watchdog has said.
Mohamed ElBaradei was speaking as the IAEA's governing board met in Vienna.
Last week, he said Iran had been more open about its past, but still enriched uranium in defiance of the UN.
Western countries are seeking further sanctions fearing Iran's programme is not peaceful. Iran says it co-operates and there is no need for sanctions.
Mr ElBaradei told the meeting the IAEA was "unable to provide credible assurance about the absence of undeclared nuclear material and activities.
"This is especially crucial in the case of Iran because of its history of undeclared activities, and the corresponding need to restore confidence in the peaceful nature of Iran's nuclear programme,"
The IAEA is meeting to discuss what to do about Iran, and in particular, whether to move towards imposing tougher sanctions on it, as some Western states want.
The US and its European allies who are permanent members of the UN Security Council - France and Britain - fear Tehran is enriching uranium in order to build nuclear weapons, and are pushing for a third round of UN sanctions.
Russia and China, the other two permanent members of the Security Council, are reluctant to pursue sanctions, saying further negotiations are needed.
Iran's ambassador to the IAEA, Ali Ashgar Soltanieh, told the meeting Tehran was co-operating with the IAEA. He warned that new UN sanctions would be counter-productive.
"We will continue this mood of co-operation provided that international community and peace-loving countries prevent the United States or others to make noise and create problems and jeopardise this constructive approach,"
"The United Nations Security Council involvement has to stop, the sooner the better."
'More co-operation needed'
The Vienna meeting provides Mr ElBaradei with an opportunity to defend his "work plan" with Iran.
That set up a mechanism for Tehran to provide more clarity about its nuclear programme.
Mr ElBaradei has faced criticism that it has given Iran the chance to delay more international sanctions.
In the latest report, Mr ElBaradei said Iran has provided more information on past aspects of its nuclear programme.
But he said more co-operation was needed to explain current activity, including traces of highly enriched uranium that inspectors have found at nuclear sites.
Uranium can be enriched for use as fuel in power plants, but more highly enriched it can be used to make weapons.
Iran insists that its programme is for civilian energy purposes only and refuses to accede to demands that it suspend uranium enrichment until international fears are dispelled.
Its top nuclear negotiator, Saeed Jalili, says he will meet European Union foreign policy chief Javier Solana in London on 30 November to discuss nuclear issues. Mr Solana's office has yet to confirm a date.