The UN atomic watchdog said that "intensive" talks Friday with Iran had failed, with no plans for a follow-up meeting to persuade Tehran to address suspected evidence of nuclear weapons research.
"Discussions today were intensive, but important differences remain between Iran and the agency that prevented agreement," International Atomic Energy Agency chief inspector Herman Nackaerts said after the talks in Vienna.
"At the moment we have no plans for another meeting," he told reporters.
Iran's envoy to the IAEA, Ali Asghar Soltanieh, however was more positive, saying that some progress had been made and that more talks would take place, although he too conceded that "differences" remained.
The meeting, the first since June and the latest in a series this year, comes as Iran faces unprecedented sanctions over its nuclear work and amid heightened speculation that Israel may bomb its arch foe's nuclear facilities.
It also came a week before the expected release of the IAEA's latest quarterly report on Iran which is expected to show that Tehran is continuing to expand its nuclear programme despite the international pressure.
Parallel, higher-level negotiations between Iran and the P5+1 -- the five permanent members of the UN Security Council plus Germany -- meanwhile appear deadlocked after three high-profile but fruitless gatherings since April.
"Issues related to the national security of a (IAEA) member state is something very delicate," Soltanieh told journalists after the more than seven-hour parley at Iran's Vienna embassy.
"I have to say that we are moving forward and this meeting in fact was an indication that we can work with the agency closely and we are going to continue this process."
As a signatory to the nuclear non-proliferation treaty -- unlike Israel, the Middle East's only if undeclared state with the bomb -- Iran's nuclear facilities are under constant IAEA surveillance and are subject to frequent inspections.
But the IAEA also wants Iran to explain indications that until at least 2003, and possibly since, Tehran carried out "activities relevant to the development of a nuclear explosive device".
It wants access to specific documents and to scientists involved in Iran's programme, as well as to sites, including the Parchin military base near Tehran, which it visited twice in 2005 but wants to look at again.
So far Iran has flatly rejected the claims, set out in a major IAEA report last November, saying they are based on forged documentation, and denied seeking -- or ever having sought -- to develop atomic weapons.
"Any information that says that Iran has nuclear weapons activities is 100 percent false and fabricated," Soltanieh told AFP after Friday's talks.
"We are at the same time ready -- and that is why we are negotiating a framework -- to work with the agency to prove it to the whole world."
Iran has said it will allow monitors access only as part of a wider arrangement governing relations between Iran and the watchdog, which experts and diplomats say would limit to an unacceptable degree the IAEA's inspection rights.
Western countries have accused Iran of bulldozing parts of Parchin to remove evidence, and the IAEA said in May that activities spotted there by satellite "could hamper the agency's ability to undertake effective verification."
One Western diplomat said they expected the IAEA to say in its report next week that Parchin has been altered so much that inspecting it now would be "irrelevant and academic."
He and other envoys think the IAEA report will say that Iran has installed but is not yet operating several hundred new centrifuges to enrich uranium to 20 percent purity at Fordo, adding to the 1,000 or so already in place there.
Enriched uranium is the main concern of the international community because it can be used not only in power generation and for medical isotopes but also, when purified to 90 percent, in the explosive core of a nuclear bomb.
The Fordo site is dug into a mountain near the holy city of Qom, making it difficult to bomb, and Iran has said it will house 3,000 centrifuges. It also has around 10,000 at its Natanz facility, to produce mostly low-enriched uranium.