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Associated Press
The Director General of the International Atomic Energy Agency, IAEA, Yukiya Amano of Japan is pictured during an interview at his office in Vienna, Austria, Tuesday, April 2, 2013.

VIENNA — The U.N.'s top nuclear official expressed concern Tuesday that Iran may be secretly continuing work on nuclear weapons while his agency is tied up in protracted negotiations with the country on restarting an investigation into past suspected research and development of such arms.

The comments by International Atomic Energy Agency head Yukiya Amano are bound to resonate with Israel and Western nations, which assert Iran is seeking nuclear weapons capacity even though it insists its atomic activities are transparent and peaceful.

At the same time, critics question the objectivity of the intelligence such assessments are based on, noting most comes from the United States and Israel, Iran's greatest detractors. They say that if the information cannot be vetted publicly it should at least be shared with Tehran so that the Islamic Republic can see the evidence used to cast suspicion upon it.

Iran denies any secret weapons work — it says its nuclear program is primarily for medical and energy purposes — and skeptics note that even the United States said in 2007 that Tehran had suspended all meaningful weapons development by 2003.

Since then, however, IAEA reports have listed suspicions of tests and experiments past that date. Britain, France, Germany and Israel have also said that such work continued beyond 2003.

"We do not know for sure, but we have information indicating that Iran was engaged in activities relevant to the development of nuclear explosive devices in the past and now," Amano said.