The head of the UN nuclear watchdog says Iran has assured him it will answer all the agency's questions about its nuclear activities and that it is also ready to sign an agreement that would allow tougher inspections.
By Jim Muir
BBC Tehran correspondent
Mohammed ElBaradei, who is in Tehran for urgent talks on Iran's nuclear programme, has been urging the Iranians to disclose fully all their nuclear activities so he will be in a position to certify that they are not trying to develop nuclear weapons.
Iran has only another two weeks to satisfy the International Atomic Energy Agency's requirements, otherwise it could be referred to the UN's Security Council for possible sanctions.
Mr ElBaradei had a long meeting with the head of Iran's Supreme National Security Council, Hasan Rohani.
He has been put in charge of the nuclear file here.
Both sides emerged apparently optimistic that serious progress will be made in the coming days as the deadline approaches for Iran to convince the agency that it has come clean over its nuclear programme.
Mr ElBaradei said he had been assured that Iran would speed up the process of answering all the questions the IAEA is asking about its activities.
He has also been told that Iran is willing to sign an additional protocol which would allow tougher agency inspections.
Negotiations on that are expected to start on Saturday, with Iran seeking assurances that its national security and sovereignty would be respected.
But for the IAEA, the top priority is to assure itself that it fully understands all Iran's past and present activities.
Mr ElBaradei seems to have come away with the impression that Iran genuinely intends to come clean, even if that means making some awkward admissions.
However actions are what counts, and everything will depend on the scope and speed of the co-operation that is forthcoming in the next two weeks.
ElBaradei sees progress in his talks
Iranian officials were also upbeat.
Iran's ambassador to the IAEA, Ali-Akbar Salehi, told the BBC he was hopeful that there would soon be the good news that concern over Iran's nuclear programme had become a thing of the past.
One of the main concerns Iran has had is the agency's request that it should suspend uranium enrichment activities, at least for the time being.
Those concerns may have been partially allayed by a low-profile visit to Tehran this week by senior officials from Britain, France and Germany.
They were following up on an earlier letter sent to Iran by those three countries indicating that they would be favourably disposed to encourage the supply of enriched uranium fuel to Iran if it complies with all the IAEA's requirements.
Before arriving in Tehran, Mr ElBaradei said that Iran had been providing more information and more access to the inspectors than before, including access to some military sites.
But he said it still was not providing the 100% transparency and pro-active co-operation that the agency needed.