A top aide to the chief of the U.N. nuclear agency has unexpectedly resigned, suggesting tensions among the organization's top leadership, diplomats said Friday.

The move by IAEA Assistant Director General Rafael Mariano Grossi comes at a critical time for the International Atomic Energy Agency. It is the outside world's only window on Iran's nuclear program, which some nations fear is close to the ability to make atomic arms — a goal Iran strenuously denies.

IAEA inspectors monitor Tehran's known nuclear facilities including its expanding uranium enrichment program, which Tehran says is meant only to produce nuclear power and for other peaceful uses. But the United States, Israel, their allies and other nations fear the Islamic Republic could use the technology to make the core of a nuclear weapon.

The agency also is trying to kick-start a probe of suspicions that Iran has secretly worked on developing nuclear weapons after more than five years of stagnation. Iran denies such work and says the allegations are based on falsified intelligence from Israel and the West. The two sides plan to resume talks on the issue in mid-May.

Two diplomats demanded anonymity in exchange for speaking The Associated Press about the resignation because they were not authorized to discuss internal IAEA matters with reporters.

One of them said Grossi told Amano he was quitting earlier this week after being told that his contract was not being extended. He said Grossi would now become the chief delegate of his country, Argentina, to the IAEA and other Vienna-based U.N. organizations. He was also expected to become Argentine ambassador to Austria.

IAEA officials said the agency had no comment.

Grossi had been widely seen as a possible successor to Amano, who was re-elected for a second term earlier this year.

A career diplomat, he had assumed an increasingly visible role over the past year, accompanying senior technical and legal experts on trips to Tehran in attempts to restart the probe into Iran's alleged secret nuclear work. His presence in the delegation was seen as a move by Amano to have more direct reporting from those trips.