The chief UN nuclear inspector said he hoped Iran would grant access during talks in Tehran on Thursday to a military base central to the IAEA's suspicions of atomic weapons research.

"We also hope that Iran will allow us to go the site of Parchin, and if Iran would grant us access we would welcome that chance and we are ready to go," Herman Nackaerts told reporters at Vienna airport on Wednesday on his way to Tehran for the meeting.

One Vienna diplomat said that the seven-man team led by Nackaerts was larger than in his last visits to Tehran in February and in May and now included two "technical experts" who could conduct verification work at Parchin -- if invited.

The International Atomic Energy Agency (IAEA) says it has evidence suggesting Iran conducted explosives research at Parchin that would be applicable in making nuclear weapons.

Iran denies seeking or ever having sought the bomb and has refused the IAEA access to Parchin, saying that as a non-nuclear site the agency has no right to conduct inspections there.

Tehran also says that the IAEA has already visited the site near Tehran twice in 2005. The agency counters that since then, it has received additional information that makes it want to go back.

The IAEA also says "extensive activities" spotted by satellite at Parchin began in January, such as the scraping and removal of earth over a 25-hectare (62-acre) area, leading to Western accusations that Iran is destroying evidence.

The alleged nuclear work at Parchin is part of a range of suspected activities summarised in a major IAEA report released in November 2011 that led to an increase in pressure on the Islamic republic.

The report said the alleged evidence was "overall, credible" and that it indicated that until 2003, and possibly since, Iran carried out work "relevant to the development of a nuclear explosive device".

Because the bulk of the information in the report comes from foreign intelligence agencies, Iran has said it is either forged or related to non-nuclear work.

The IAEA has zeroed in on Parchin because some of its information on the base is "independent", including satellite images and from a "foreign expert" named in media reports as former Soviet nuclear scientist Vyacheslav Danilenko.

Thursday's meeting between Nackaerts' team and Iranian officials is the latest in a string of fruitless talks this year between the Iranians and the IAEA, the latest in August in Vienna.

The United States has warned that it will push for the board of the Vienna-based agency to refer Iran to the UN Security Council if Tehran displays no "substantive cooperation" by March.

Nackaerts said Wednesday that efforts to agree a "structured approach" to resolve the outstanding issues had been "under negotiation now for almost one year."

Recent rhetoric from Iran has not however indicated that Iran has become any more conciliatory, however, despite huge sanctions pressure and the looming threat of military action by Israel.

Mark Fitzpatrick from the International Institute for Strategic Studies in London told AFP earlier that progress on Thursday would be a "pleasant surprise."

Watching closely will be six major powers keen to restart diplomatic efforts to resolve the decade-long and escalating crisis over Iran's nuclear programme.

Those efforts are focused more on Iran's current activities, in particular its expanding ability to enrich uranium to fissile purities of 20 percent.

Multiple UN Security Council resolutions have called on Iran to suspend all enrichment because the IAEA, which closely monitors Iran's nuclear sites, says it is "unable" to conclude all activities are peaceful.

The six powers -- the US, Russia, China, Britain, France and Germany -- are thought to be discussing possible changes to an offer rejected by Iran in their last round of talks in Moscow in June.

Popular US Middle East issues website www.al-monitor.com reported this week that Russia had not yet agreed to an "updated" package of proposals because it did not offer Iran sufficient sanctions relief.