Iranian officials in Tehran have begun negotiations with International Atomic Energy Agency (IAEA) envoys on signing an agreement to allow tougher inspections of its nuclear facilities.
The IAEA has urged Iran to sign the Additional Protocol to the Nuclear Non-Proliferation Treaty as part of a process of establishing that Tehran has no intention of trying to produce nuclear weapons.
The Iranians have until the end of the month to answer all the agency's questions about their past nuclear activities.
Tehran denies it has a nuclear weapons programme
The negotiations over the Protocol are a vital part of a complex package which is being put together.
If all goes well, it could lead to Iran taking a big step towards shrugging off US President George W Bush's "axis of evil" stigma, and significantly improving its relations with Europe.
Iranian officials say the talks on the Protocol may go on for several days. But the IAEA negotiators expect to be in Tehran only until Sunday, so it could be sooner.
If all goes well, and Iran signals that it is ready to announce acceptance of the protocol and meet other requirements, the foreign ministers of Britain, France and Germany are expected to fly to Tehran at very short notice.
Their purpose would be to assure Iran that if it signs the protocol and meets all the IAEA's other requirements, they would help ensure that the Iranians get access to the technology and enriched uranium fuel they need to produce their own peaceful nuclear power.
The ministers will only come if that understanding is assured.
Diplomats say it would depend on Iran's agreeing not only to sign the protocol, but also to suspend uranium enrichment activities as the IAEA has requested, and later to restrict them, under international safeguards, to low-grade fuel production sufficient for producing atomic power but not nuclear weapons.
The head of the IAEA, Mohamed ElBaradei left Tehran late on Thursday after receiving assurances that Iran was ready to come completely clean over its past nuclear activities, as demanded by a tough resolution from the Agency's Board of Governors in September which set Iran a deadline expiring at the end of October.
By then, the Iranians would have to clarify many outstanding questions, especially in the area of uranium enrichment.
Mr ElBaradei hinted that that might mean Tehran making some potentially embarrassing admissions about its past activities.
Iran faces a vital turning point. While the signals at present are positive, if things go wrong, it could lead in quite the opposite direction.
If by the end of the month Iran has failed to give the IAEA all the information it needs to answer all the questions, it could find itself referred to the UN Security Council for possible sanctions.
That course would also entail, among other things, a huge setback to Tehran's relations with Europe.
The European Union has made it clear that continuing its current negotiations for a Trade and Cooperation Agreement with Iran depends heavily on Tehran's complying with the IAEA's requirements.