Iran has been urged to sign up to strengthened international inspection of its nuclear activities by the head of the UN's nuclear watchdog and the European Union.
The IAEA wants more access to Iran's suspected sites
Mohammed ElBaradei told the International Atomic Energy Agency (IAEA) that Iran had failed to report some of its nuclear activities, though he said it was now taking steps to rectify the situation.
He called on Tehran to sign an additional protocol to the Nuclear Non-Proliferation Treaty, which would allow the IAEA to inspect all nuclear sites in the country.
The Iranian representative at the meeting said the issue had been "politically motivated and politically charged", but would be resolved.
And in Tehran, a spokesman for the country's Atomic Energy Organisation said Iran was studying the call to sign an additional protocol "with a positive view".
Mr ElBaradei said that Tehran's co-operation would enable the IAEA "to provide credible assurances regarding the peaceful nature" of Iran's nuclear programme.
Allows for inspections at short notice
IAEA can take environmental samples at any location
The United States believes that Iran is pursuing a covert weapons programme.
The US ambassador to the IAEA, Kenneth Brill, said that the meeting in Vienna would be "very serious and sobering".
Earlier, the Iranian foreign ministry had rejected calls by the European Union for Iran to sign a new protocol "urgently and unconditionally".
EU foreign ministers meeting in Luxembourg are expected to say that improved trade links with Iran should be conditional on such a move.
But the linkage was repudiated by Tehran.
"We will not accept any preconditions in our negotiations." said foreign ministry spokesman Hamid Reza Asefi.
Some EU countries want trade talks with Iran halted, but a majority believe the EU should keep the door open to dialogue, as a means of obtaining greater transparency on nuclear issues and more progress on human rights and political reforms in Iran.
"We've been negotiating a trade and cooperation agreement with
Iran which we wish to see in place," said UK Foreign Secretary Jack Straw.
"But it is essential too that we see good progress on non-proliferation compliance."
Former President Ali Akbar Hashemi Rafsanjani, who is still powerful in the government, said on Sunday that Iran's foreign policy was "purely defensive" with "no policy of aggression in the Islamic Republic".
"It is our right to benefit from atomic energy," he said.
The IAEA report had been leaked to the media last week.
It says that Iran has failed to:
Mr ElBaradei visited Iran in February, and toured a nuclear plant under construction at Natanz, 320km (200 miles) south of Tehran.
The site is crucial, because it is where Iran is developing a series of centrifuges, which could be used to produce enriched uranium - the material used for making a nuclear bomb.
The IAEA reports says that Iran's failure to provide information in a timely manner has become "a matter for concern".