IAEA director-general ElBaradei gives Tehran four weeks to clear up questions over nuclear programme.
The UN atomic watchdog announced on Sunday that Iran has agreed to clear up remaining questions on its nuclear programme - including any military activity - in four weeks.
International Atomic Energy Agency (IAEA) director-general Mohamed ElBaradei, and Iran's supreme leader Ayatollah Ali Khamenei and president Mahmoud Ahmadinejad agreed that a "work plan" on ending the Iran nuclear standoff should be completed in four weeks, the agency said in a statement.
The deputy chief of Iran's atomic energy agency, Mohammed Saidi, confirmed the timeframe, the country's state news agency IRNA reported.
"Iran will respond within the space of four weeks to the remaining questions so that the IAEA can make a transparent report on the Iranian nuclear programme," said Saidi.
"The Islamic republic of Iran has nothing to hide, and that's why it does not fear answering the remaining questions. I am optimistic."
However, diplomats in Vienna, where the IAEA is based, expressed scepticism that Iran would come clean about its nuclear activities within the new timeframe. And the US said the agreement does not go far enough, insisting the Islamic republic suspend uranium enrichment.
"Answering questions about their past nuclear activities is a step, but they still need to suspend their enrichment and reprocessing activity," said a White House spokesman.
Earlier, US president George W. Bush in a speech in Abu Dhabi branded Iran "the world's leading state sponsor of terror".
ElBaradei, his deputy Olli Heinonen, and the agency's head of external relations, Vilmos Cserveny, were in Tehran on Friday and Saturday, where they met Khamenei and Ahmadinejad.
The work plan was part of a deal struck between ElBaradei and Tehran last year to deal with unresolved questions on Iran's atomic drive.
The key issues were Iran's past experiments with plutonium, its use of uranium-enriching P1 and P2 centrifuges, questions about particles of arms-grade enriched uranium found by IAEA inspectors at Tehran's Technical University, and most significantly possible military applications of the nuclear technology.
Originally, the work plan had envisaged resolving all issues by the end of 2007.
In Tehran, the head of Iran's atomic energy organisation Gholam Reza Aghazadeh had told the ISNA news agency: "We are hoping that all the past and present questions about our dossier will be solved and that we will return to a normal situation."
The IAEA said the talks in Tehran were on "ways and means to accelerate implementation of safeguards in Iran, as well as additional confidence-building measures...".
"According to the agreed schedule, implementation of the work plan should be completed in the next four weeks," it said.
But Western diplomats said Iran had long been aware of the issues which worry the UN and that it had originally agreed to clear them up by the end of last year.
"So it's about time it got on with resolving them rather than just stringing the process out," said a British diplomat, on condition of anonymity.
"Fundamentally, the issue is one of confidence and only by suspending [uranium enrichment] to allow us to enter into negotiations will we be able to work out a long-term solution to provide that confidence," the diplomat said.
Another Western diplomat said the work plan was just "one small part" of any overall settlement with Iran. "Suspension of enrichment and applying the additional protocol are perhaps even more important."
Enriched uranium is used to make both nuclear fuel and nuclear weapons and the West has always insisted that Iran suspend enrichment to prove to the international community that its nuclear programme is peaceful, as Tehran claims.
Iran insists that it has an inalienable right to develop nuclear power for a growing population with increasing energy needs and adamantly refuses to suspend enrichment, in defiance of UN Security Council resolutions.
Sunday's IAEA statement made no specific mention of the enrichment question.