The UN atomic agency will on Wednesday press Iran, in their 10th meeting since late 2011, to grant access to sites, documents and scientists involved in Tehran's alleged efforts to develop nuclear weapons.

The International Atomic Energy Agency (IAEA) says that there is "overall, credible" evidence that until 2003, and possibly since, such activities took place.

The agency conducts regular inspections of Iran's declared nuclear facilities and Tehran, which denies seeking or ever having sought atomic weapons, says it is not obliged to grant access to any other sites.

Iran says the IAEA's findings are based on faulty intelligence from foreign spy agencies such as the US CIA and Israel's Mossad -- intelligence it complains it has not even been allowed to see.

Nine rounds of talks, the latest in February, since the publication of a major IAEA report in November 2011 on these alleged activities have produced no breakthrough. Iran says progress has been made but the agency denies even this.

In particular, the IAEA wants to be allowed to go to the Parchin military base near Tehran. It says its information on alleged activities at the site are not from spy agencies and can therefore be easily shared.

Iran counters that the IAEA already visited Parchin twice in 2005. The agency says that it has obtained new information since then, and Western countries have accused Iran of hiding evidence there.

Ali Asghar Soltanieh, Iran's ambassador to the IAEA, has called such accusations "childish".

In March, IAEA head Yukiya Amano called on Iran to grant access to Parchin without waiting for a wider accord covering all outstanding issues, but Tehran rejected the idea.

Parallel diplomatic efforts meanwhile between Iran and six major powers -- the US, China, Russia, Britain, France and Germany -- are focused more on Iran's current activities, most notably uranium enrichment.

Enriched uranium is at the heart of the international community's concerns since it can be used not only for peaceful purposes such as power generation but also -- when highly purified -- in a nuclear bomb.

The latest round with the "P5+1" in Almaty, Kazakhstan in early April ended with chief negotiator and EU foreign policy head Catherine Ashton saying the two sides remained "far apart".

The UN Security Council has passed multiple resolutions calling on Iran to suspend all uranium enrichment, imposing several rounds of sanctions on the Islamic republic.

Additional US and EU sanctions last year began to cause major economic problems by targeting the Persian Gulf country's vital oil sector and financial system.

Israel, the Middle East's sole if undeclared nuclear-armed state, meanwhile has refused to rule out military action on Iran, as has Washington.

The IAEA talks come ahead of the release next week of its latest regular report, which is expected to show that Tehran has continued to expand its nuclear programme in spite of its international isolation.

"It seems like Iran is trying one more time to offer a minimal conversation right before the (IAEA) director general issues his next report ... in a calculated move by Iran to soften criticism," one Western diplomat told AFP.

Mark Fitzpatrick, analyst at the International Institute for Strategic Studies in London, was similarly pessimistic.

"The IAEA is persistent in asking for a means of resolving the issues, but Iran is stubbornly insisting on tying its IAEA obligations to sanctions relief on the diplomatic track," he told AFP last month.