The UN atomic watchdog and Iran appeared to make little real headway in talks last week and it is uncertain whether Tehran's more positive attitude will help yield a long-sought breakthrough, Reuters cited diplomats as saying on Wednesday, Oct 2.

Iran told the International Atomic Energy Agency in a meeting it wanted to achieve substantive results within months in the talks on a stalled IAEA inquiry into suspected atomic bomb research by the Islamic state, one envoy said.

But he and others briefed on the closed-door discussions stressed that hopes had been raised before in Iran-IAEA meetings since early 2012, only to be dashed by what Western states saw as Iranian stonewalling. Iran denies any nuclear weapon aims.

One Western diplomat said he had the impression that Iran and the IAEA were relatively "optimistic" after the meeting in Vienna, where the UN agency is based. Another envoy said the discussions had been focused and the atmosphere positive.

Both sides, including the new head of Iran's delegation, described their discussions as "constructive" and said the next meeting would be held on October 28 but gave no detail.

The IAEA talks are distinct from Iran's meetings with world powers, but both diplomatic tracks center on suspicions that Iran may be seeking the capability to assemble nuclear bombs behind the facade of a civilian atomic energy program.

The Iran-IAEA meeting was a "good harbinger of better relations", said Mark Fitzpatrick of the International Institute for Strategic Studies, a London-based think tank.

"There is a new mood of optimism in Vienna that finally there is a way forward," Fitzpatrick said.

The IAEA wants access to sites and officials for its probe into what it calls the possible military dimensions to Iran's nuclear program. Iran has dismissed allegations that it may have worked on designing a nuclear bomb as forged or baseless.

Eleven meetings since January last year have failed to end the deadlock over how the IAEA should conduct the investigation.

But Iran has pledged, since Rouhani took office in August, to increase its cooperation with the IAEA and it appointed a new envoy to the UN agency in August.

The next IAEA talks will take place about two weeks after Iran meets six world powers in Geneva in mid-October.

Western diplomats have long suspected that Iran might only agree to provide the IAEA with the access it wants as part of a broader settlement that wins it significant sanctions easing in return for scaling back its nuclear program.

The six powers - the United States, France, Britain, Germany, China and Russia - "have to make sure that their desire to solve this crisis once and for all doesn't sacrifice" the IAEA's investigation, nuclear expert Mark Hibbs of the Carnegie Endowment think tank said.

"The IAEA can't simply ignore what Iran has done in the past because it is politically expedient," Hibbs said.