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VIENNA, Austria - A senior U.N. nuclear agency team will visit Tehran on Jan. 28 with Iran saying it is ready after years of refusal to discuss allegations that it was involved in secret nuclear weapons work, diplomats said.

Diplomats have previously said that International Atomic Energy Agency officials were discussing such a trip with their Iranian counterparts. But before the diplomats' comments Thursday, no date -- or indication that Iran was ready to talk about the allegations -- had been mentioned.

Any follow-through on the part of Iran on its reported pledge to discuss nuclear arms suspicions would be significant.

For more than three years, Tehran has blocked IAEA attempts to follow up on U.S. and other intelligence alleging covert Iranian work on nuclear arms, dismissing the charges as baseless and insisting all its nuclear activities were peaceful and under IAEA purview.

Faced with Iranian stonewalling, the IAEA summarized its body of information in November, in a 13-page document drawing on 1,000 pages of intelligence. It stated then for the first time that some of the alleged experiments can have no other purpose than developing nuclear weapons.

Iran continues to deny the charges and no change in its position is expected during the Tehran talks with IAEA officials. But even a decision to enter a discussion over the allegations would be a major departure from outright refusal to talk about them -- and create hopes of future progress in the investigation.

Two diplomats told The Associated Press that Iranian officials had suggested they were ready to talk about the issue during recent meetings with officials of the Vienna-based IAEA. They asked for anonymity because their information was confidential.

Ali Asghar Soltanieh, Iran's chief IAEA delegate, declined to be drawn on what would be discussed in Tehran, indicating in comments to The AP that it was too early to go public with details.

The composition of the IAEA team, as described by one of the diplomats, also reflects the significance the U.N. nuclear monitoring agency attaches to the visit, which is expected to conclude in the first few days of February.

Normally such trips are made by Deputy Director General Herman Nackaerts, in charge of the Iran nuclear file, and more junior officials of his department. But the diplomat said that this time Nackaerts will be accompanied by Assistant Director General Rafael Grossi, the right-hand to IAEA chief Yukiya Amano, and Peri Lynne Johnson, the agency's senior legal official.

Johnson is the only American among the three. While IAEA officials are formally neutral, her citizenship is of potential significance considering the high tension-level between Washington and Tehran.

Beyond the dispute over Iran's nuclear intentions, U.S.-Iranian relations have been further burdened by an Iranian announcement that a joint U.S.-Iranian national will be executed after being found guilty of spying -- a charge both he and Washington denies.

Iran, in turn, sees possible U.S. complicity in a series of assassinations of its nuclear experts -- the latest Wednesday, when scientist Mostafa Ahmadi Roshan was killed by a bomb attached to his car by a passing bicyclist.

In a letter to the U.N. secretary-general on Thursday, Iran's U.N. Ambassador Mohammad Khazaee called on the U.N. to condemn the killing and two earlier attacks that left two nuclear scientists dead and another seriously injured.

"There is firm evidence that certain foreign quarters are behind such assassinations. As has been claimed by these circles, such terrorist acts have been carried out as part of the efforts to disrupt Iran's peaceful nuclear program, under the false assumption that diplomacy alone would not be enough for that purpose," the letter read in part.

U.S. Secretary of State Hillary Rodham Clinton denied any U.S. role in the slaying and the U.S administration condemned the attacks. Israeli officials, in contrast, have hinted at covert campaigns against Iran without directly admitting involvement.

Beyond urging Iranian co-operation with the IAEA probe of the alleged weapons work, the U.S. and its allies are pressuring Iran to halt uranium enrichment, a key element of the nuclear program that dozens of nations suspect is aimed at producing atomic weapons. Uranium enriched to low levels can be used as nuclear fuel, but at higher levels it can be used as material for a nuclear warhead.

Iran denies it is trying to make nuclear weapons, saying its program is for peaceful purposes only and is geared toward generating electricity.

Those claims were called into question on Monday when the IAEA confirmed Iran had begun increasing its production of uranium enriched to 20 per cent. That's a significantly higher concentration than the nation's main stockpile -- and can be turned into weapons-grade material more quickly than the lower enriched uranium.

Olli Heinonen, Nackaert's predecessor, noted that "if Iran decides to produce weapons-grade uranium from 20 per cent enriched uranium, it has already technically undertaken 90 per cent of the enrichment effort required."

"This does not automatically mean Iran will be able to build a nuclear weapon in one month -- building an atomic bomb is a complex endeavour that requires precision engineering capabilities that Iran may lack," wrote Heinonen, in a commentary for Foreign Policy magazine. "But it does mean that the country would be able to 'break out' of its international obligations very quickly should it decide to do so."