As International Atomic Energy Agency chief Mohamed ElBaradei was preparing to fly to Tehran for a rare visit at the end of the week, IAEA officials were already "on the ground" in the Iranian capital "doing some important leg work," one diplomat said on condition of anonymity.

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 a number of outstanding issues about the Islamic republic's atomic drive.

These included 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 of the issues by the end of 2007.

With the so-called contamination issue the focus of discussions held in November, next on the list was, according to the plan, "activities that could have military applications."

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

On Monday, the IAEA announced that ElBaradei had been invited to Tehran this Friday and Saturday, where he would "meet with a number of high officials."

Diplomats here suggested those officials could include Iran's supreme leader Ayatollah Ali Khamenei and President Mahmoud Ahmadinejad.

The aim, according to IAEA press spokeswoman Melissa Fleming, was "to enhance and accelerate implementation of safeguards in Iran, with a view of resolving all remaining outstanding issues and enabling the agency to provide assurance about Iran's past and present nuclear activities."

Despite four years of investigation, the IAEA has never been able to confirm if Iran's nuclear drive is peaceful.

The US intelligence community last year issued a report which said Iran had had a nuclear weapons drive, but halted it in 2003.

In its own report published last November, the Vienna-based UN watchdog complained that while Iran had taken important steps in revealing the extent of its nuclear programme, it was still defying UN demands to suspend uranium enrichment.

Contacted by AFP on Tuesday, the US ambassador to the IAEA, Gregory Schulte, said he believed ElBaradei was travelling to Tehran this week "to obtain full disclosure from Iran about its nuclear programmes."

For Washington, the "central issue" was "obtaining from Iran an admission of the entire history of its nuclear weapons programme and verification that the programme has stopped," Schulte said.

ElBaradei's pending trip to Tehran was "an opportunity to press for a full confession (on the part of Iran) and subsequent verification without delay," he said, adding that the US supported ElBaradei's efforts.

Along with the other countries that hold permanent seats on the UN Security Council, the US would "continue to take additional measures to pressure Iran to meet its international nuclear obligations, in particular, the requirement that Iran suspend its uranium enrichment and other proliferation-sensitive nuclear activities," Schulte said.

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