Iran is to respond Tuesday to a call by six world powers to suspend uranium enrichment, the process that makes nuclear power reactor fuel but also raw material for atom bombs.

If Iran refuses, UN sanctions could follow.

Iran has recently filed a verbal complaint with the Vienna-based International Atomic Energy Agency, which has been investigating Tehran's nuclear program for over three years, about an IAEA expert "acting outside the responsibilities of an inspector," a senior Western diplomat close to the IAEA said.

This apparently involves comments the inspector made while in Iran recently and even alleged spying activities but this could not be confirmed.

Iranian officials could not immediately be reached for comment.

The diplomat, and a second source close to the IAEA, both requesting anonymity due to the sensitivity of the issue, said Iran had "withdrawn the designations" of two other agency inspectors, in March and April.

There had been no such previous incidents since the IAEA began its investigations in Iran in February 2003, the diplomats said.

A third diplomat said the "Iranians are posing a lot of problems to inspectors" as deadlines fall for it to rein in its nuclear activities.

A fourth, Middle Eastern diplomat, said: "The Iranians are showing that they will not respond under duress and they think they'll get away with it."

The IAEA verifies compliance with safeguards guarantees mandated under the nuclear Non-Proliferation Treaty (NPT), checking on whether nuclear material is being diverted for non-peaceful purposes.

For instance, the IAEA monitors activity at a uranium enrichment facility in Natanz in Iran.

The first diplomat said that while Iran's recent actions do not make the IAEA's job easier, it was "not a crisis because the IAEA can still do its inspections."

"It would be a crisis if the agency were unable to verify (Iran's nuclear activities) and we are a long way from that," the diplomat said.

Iran cut down on IAEA access once the agency had referred it to the United Nations Security Council earlier this year, ceasing to apply an Additional Protocol to the NPT that allowed for wider, short-notice inspections.

But it is still applying the Safeguards Agreement.

The IAEA "designates" inspectors to countries with which it has a safeguards agreement, and countries are then free to accept, or not, these experts.

"The IAEA has more than 200 inspectors designated for Iran," the first diplomat said, adding that the actual investigative work is carried out by "a team of about 20."

The inspectors who have been rejected were "a non-European Caucasian" in March, and in April, the Belgian Chris Charlier, who is the IAEA's chief inspector for Iran, diplomats said.

The Iranians said Charlier had been talking to the media and had made unauthorized tape recodings, diplomats said.

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