International Atomic Energy Agency chief Mohamed ElBaradei is due in Tehran on Friday and Saturday in an attempt to bring the IAEA's four-year investigation into Iran's atomic activities closer to conclusion.

The aim, according to ElBaradei's spokeswoman Melissa Fleming, was to resolve "all remaining outstanding issues and (enable) the agency to provide assurance about Iran's past and present nuclear activities."

The IAEA has never been able to confirm once and for all that Iran's nuclear drive is entirely peaceful, as Tehran claims.

But under a so-called "work plan" agreed between ElBaradei and Tehran last year, Iran and IAEA officials have been holding a series of discussions to resolve the key outstanding issues.

These include Iran's past experiments with plutonium, its use of uranium-enriching P1 and P2 centrifuges, and questions about particles of arms-grade enriched uranium found by IAEA inspectors at Tehran's Technical University.

Originally, the work plan had envisaged resolving all issues by the end of 2007, but that deadline has since passed uncommented by all sides.

Nevertheless, Vienna-based diplomats said talks had indeed turned to the last and, possibly most significant, item on the list, that of the possible military use of Iran's nuclear technology.

In Tehran on Wednesday, the INSA news agency reported that the previous issue, the so-called contamination issue, had been wrapped up, enabling investigators to move forward to the next one -- that of possible military applications.

While the IAEA insists that no issue is ever "closed" once and for all, the Iranian side is adamant that each topic must be treated sequentially.

The military issue is particularly significant given the West's suspicions that Iran has been seeking to develop the atomic bomb.

ElBaradei has previously made five trips to Iran since the nuclear standoff between the Islamic Republic and the international community began in 2002.

This time, ElBaradei will be accompanied by his number two at the IAEA, safeguards chief Olli Heinonen, and the agency's director of external relations, Vilmos Cserveny.

The IAEA was predictably tight-lipped about the itinerary and would only say that ElBaradei would meet with "high-ranking officials."

According to diplomats here, they could include Iran's supreme leader Ayatollah Ali Khamenei and President Mahmoud Ahmadinejad.

INSA reported that ElBaradei would also meet Foreign Minister Manouchehr Mottaki, Iran's chief nuclear negotiator Saeed Jalili and the head of Iran's atomic energy organisation, Gholam Reza Aghazadeh.

Contacted by AFP on Wednesday, the Iranian mission to the IAEA said that ambassador Ali Asghar Soltanieh was currently in Tehran and was scheduled to return to Vienna later this week.

A recent US intelligence report has helped turn down the heat somewhat after it found that Tehran halted its covert nuclear weapons drive back in 2003.

Nevertheless, the US and its western allies argue the findings of the National Intelligence Estimate (NIE) make it all the more imperative for Iran to come clean about its nuclear activities, both past and present.

US-Iran tensions were high again this week following an incident between Iranian vessels and US warships in the strategic Strait of Hormuz at the weekend. And US President George W. Bush has embarked on a Middle East tour.

Daryl Kimball, executive director of the Arms Control Association in Washington, said he expected ElBaradei to try and persuade Tehran to cooperate more fully and open up its nuclear facilities to unlimited examinations.

The Egyptian-born diplomat could also "possibly float some ideas about how to resume negotiations and suspend enrichment activities," the expert suggested.

To the alarm of the US and its western allies, Iran has steadfastly refused to suspend all uranium enrichment-related activities, in defiance of UN Security Council resolutions.

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