Iran has submitted a "comprehensive" declaration on its nuclear programme, the UN nuclear watchdog says.
Tehran denies it has a nuclear weapons programme
The International Atomic Energy Agency has given Tehran until Friday to prove it is not building nuclear weapons as the United States alleges.
But its inspectors could take some time longer to check out the information which Iran has provided.
Last week, Iran promised to halt uranium enrichment and allow tougher UN inspections of its nuclear facilities.
"I think we are making good progress. Iran has submitted what [it] assured me to be a comprehensive and accurate declaration," IAEA head Mohamed ElBaradei told reporters in the Canadian
"I think I could say that at first glance the report is comprehensive but we still have to do a lot of fine-tuning, we still have to do a lot of questioning."
ElBaradei still has questions to ask
"We have our inspectors in Iran now, verifying that declaration," the IAEA head said.
But the BBC's Jim Muir in Tehran says this process will take time and will not be finished in the next day or two.
Some issues may still be unsettled when the IAEA's board of governors assembles on 20 November to assess Iran's compliance.
IAEA inspectors are understood to be focusing on processes of uranium conversion, laser enrichment and centrifuges where traces of highly-enriched weapons-grade uranium were found earlier.
Tehran also agreed to sign an additional protocol of the Nuclear Non-Proliferation Treaty (NPT), allowing UN inspectors to carry out spot checks of Iranian facilities.
The signs are that this was a major strategic decision endorsed by the main pillars of the Islamic regime, our correspondent says.
But this does not necessarily mean that things will go smoothly, and if it emerges that Iran tried to cover up past activities Mr ElBaradei will be obliged to make a tough report to his board.
'Matter of months'
As the IAEA chief welcomed Iran's co-operation, he also called for the NPT to be strengthened.
He told the French newspaper Le Monde newspaper that between 35 and 40 countries now had the ability to create nuclear weapons.
The current treaty did not control uranium enrichment or the possession of military-grade nuclear material, he said.
He added that if any of these countries decided to break their commitment to the treaty they could produce a weapon in a matter of months.
"We are already on the verge of a catastrophe with North Korea," he said, referring to Pyongyang's believed possession of nuclear bombs and threat to test one.
The NPT dates back to 1970, when only a small number of states were thought to be capable of getting nuclear weapons.