The chief UN nuclear inspector returned Friday from Iran without a hoped-for deal on investigating its nuclear programme, dampening hopes of progress in renewed talks with world powers.

Instead all a team led by Herman Nackaerts of the International Atomic Energy Agency returned to Vienna with was an arrangement to meet in the Iranian capital again on 12 February.

"Differences remain so we could not finalise the structured approach to resolve the outstanding issues regarding possible military dimensions of Iran's nuclear programme," Nackaerts told reporters at Vienna airport.

"We have agreed with Iran that we will meet again on February 12 (in Tehran)," he said.

The IAEA conducts regular inspections of Iran's declared nuclear facilities but it also wants access to what it believes are sites where undeclared activities aimed at developing nuclear weapons took place until 2003, and possibly since.

Nackaerts had said in December after an earlier visit to Tehran that he had expected, after a year of trying, to sign this week a "structured approach" deal that would see Iran answer the allegations against it.

Nackaerts said that during the talks "also on this occasion no access was granted to Parchin", one of the sites the agency would like to visit.

Iran denies ever having worked on nuclear weapons and says that the IAEA's evidence is based on faulty intelligence that it has not even been allowed to see.

It says that because no nuclear activities took place at Parchin, the IAEA has no business conducting inspections there and that it already went there twice in 2005.

It was unclear what went wrong this time but in the past Iran has insisted that the agreement include clauses that could infringe on the IAEA's ability to conduct proper inspections.

This included for example the IAEA agreeing to "close" an issue for good once it had been covered, even if new information came to light, or the agency being able to inspect a site or a document only once, diplomats said.

"It's disappointing, but not all that surprising, that Iran has yet again failed to follow through on earlier indications of flexibility," said Mark Fitzpatrick from the International Institute for Strategic Studies.

The talks with the IAEA came ahead of a new meeting between Iran and the P5+1 powers -- the US, China, Russia, Britain, France and Germany.

The meeting was expected to be held in January but is taking longer than envisaged to arrange, either because Iran was waiting until after the IAEA talks or because Tehran and the six powers are having problems agreeing an agenda.

This parallel effort is focused more on Iran's current activities, in particular uranium enrichment, a process that can be used for peaceful purposes but also for creating the core of a nuclear bomb.

At their last meeting, held in Moscow in June, Tehran rejected P5+1 calls for it to scale back its nuclear enrichment activities, while asking for substantial sanctions relief.

Iran's economy is struggling to cope with punitive measures adopted by the US and the EU targeting its vital oil income and access to global financial systems.

"I believe some of the actors in Tehran really do want to strike a deal with the IAEA as a prelude to the more sensitive talks with the P5+1," Fitzpatrick told AFP.

"But getting the political forces in Tehran in alignment on what might look like a compromise is proving to be too difficult. Uncompromising hardliners remain in command."