The news from International Atomic Energy Agency (IAEA) Chief Yukiya Amano, who returned from Tehran on Tuesday, comes just a day before Iran and six other world powers meet in Baghdad for negotiations and could present a significant turning point in the heated dispute over Iran's nuclear intentions.
The six nations hope the talks will result in an agreement by the Islamic Republic to stop enriching uranium to a higher level that could be turned quickly into the fissile core of nuclear arms.
Iran denies that it seeks nuclear arms and says that its reactors are only for power and medical applications.
By compromising on the IAEA probe, Iranian negotiators in Baghdad could argue that the onus was now on the other side to show some flexibility and temper its demands.
Although Amano's trip and the talks in Baghdad are formally separate, Iran hopes progress with the IAEA can boost its chances Wednesday in pressing the US and Europe to roll back sanctions that have hit Iran's critical oil exports and blacklisted the country from international banking networks.
It was unclear, though, how far the results achieved by Amano would serve that purpose, with his trip failing to seal a deal, despite his upbeat comments.
After talks in Tehran between Amano and chief Iranian nuclear negotiator Saeed Jalili, "the decision was made... to reach agreement" on the mechanics of giving the IAEA access to sites, scientists and documents it seeks to restart its probe," Amano told reporters at Vienna airport after his one-day trip to Tehran.
Amano said differences existed on "some details," without elaborating but added that Jalili had assured him that these "will not be an obstacle to reach agreement." He spoke of "an almost clean text" that will be signed soon, although he could not say when.
For the six powers - the United States, Russia, China, Britain, France and Germany - a main concern is Iran's production of uranium enriched to 20 percent, which is far higher than needed for regular energy-producing reactors but used for one Iran says it needs for medical research.
The US and its allies fear the higher-enriched uranium could be quickly boosted to warhead-grade material.
US officials have said that Washington will not backpedal from its stance that Iran must fully halt uranium enrichment.
But speculation is increasing that the priorities have shifted to block the 20 per cent enrichment and perhaps allow Iran to maintain lower-level nuclear fuel production - at least for now.
Iranian officials could package such a scenario as a victory for their domestic audience. In Israel, it would likely be greeted with dismay and widen rifts between President Barack Obama's US administration and Israeli officials who keep open the threat of military action against Iran's nuclear sites.
Israeli Prime Minister Benjamin Netanyahu has warned against concessions, saying world powers should make "clear and unequivocal demands" that Iran stop all of its nuclear enrichment activity.
"Iran wants to destroy Israel and it is developing nuclear weapons to fulfill that goal," Netanyahu said at a conference in Jerusalem. "Against this malicious intention, leading world powers need to display determination and not weakness. They should not make any concessions to Iran."
Jalili, Iran's top nuclear negotiator who met with Amano and will also be the lead envoy at the Baghdad talks, said his country hopes for a new beginning when the talks start on Wednesday.
"We hope that the talks in Baghdad will be a kind of dialogue that will give shape to ... cooperation," Jalili said after arriving in Baghdad late Monday.
As part of any agreement, Amano and his agency are focused on getting Iran to let agency experts to probe various high-profile Iranian sites, including the Parchin military complex southeast of Tehran, where the agency believes Iran in 2003 ran explosive tests needed to set off a nuclear charge. The suspected blasts took place inside a pressure chamber.
Iran has never said whether the chamber existed, but describes Parchin as a conventional military site. Iran, however, has blocked IAEA requests for access to sites, scientists and documents needed for its investigation for more than four years.
Amano's talks included Jalili as well as Iran's foreign minister and other officials including the head of Iran's nuclear agency, Fereidoun Abbasi.
Iranian lawmaker Heshmatollah Falahtpisheh told The Associated Press on Monday that Tehran will likely accept more inspections of Parchin "if it feels there is good will within the (IAEA)."
But Falahtpisheh warned that this new openness will likely come with expectations that the West would in return ease international sanctions on Iran.
"In opening up to more inspections, Iran aims at lowering the crisis over its nuclear case," he said. "But if the sanctions continue, Iran would stop this."
A political analyst in Tehran, Hamid Reza Shokouhi, said Iran is carefully watching to see if the West shows more "flexibility and pays attention to Iranian demands" during Amano's trip.
"Then Iran will show flexibility, too," Mr Shokouhi said.
But some Iranian media was critical of Amano and the IAEA, possibly reflecting internal divisions on how far to go compromise on nuclear issues.
In a sign of ebbing market worries, oil prices have steadily fallen since Iran and world powers resumed talks in April in Istanbul. Fears of supply disruptions because of military conflict or Iranian shipping blockades helped drive prices above $106 a barrel earlier this year. Oil rose to slightly above $92 per barrel Monday in New York.