VIENNA (Reuters) - A top U.N. nuclear watchdog official will go to Iran on Thursday, the agency said, amid tension between Tehran and big powers over its failure to address demands that it freeze sensitive nuclear work.

A spokesman for the International Atomic Energy Agency could not specify the purpose of the visit by Olli Heinonen, the IAEA's deputy director overseeing inspections and long-running U.N. inquiries into Iran's nuclear program.

The semi-official Iranian news agency ISNA said Heinonen would be in Iran for three days and that his trip was part of the U.N. watchdog's monitoring program.

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