Iran is not being completely open about its nuclear program, the head of the International Atomic Energy Agency told CNN on Wednesday.

"We have the indication or information that Iran has engaged in activities relevant to the development of nuclear explosive devices," Yukiya Amano told CNN.

Iran is not being completely open about its nuclear program, the head of the UN nuclear watchdog agency said.

"Iran is not telling us everything. That is my impression. We are asking Iran to engage with us proactively, and Iran has a case to answer," Amano told CNN.

In a videoed interview, Amano said Iran has declared a number of nuclear facilities to the IAEA, which has them under its safeguards. "For these facilities and activities, I can tell that they are in peaceful purpose," Amano said.

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However, the IAEA chief noted, Iran may have also engaged in activities linked to the development of nuclear weapons. "There may be other facilities which are not declared, and we have the indication or information that Iran has engaged in activities relevant to the development of nuclear explosive devices."

Also on Wednesday, diplomats told the Associated Press that spy satellite images of an Iranian military facility show trucks and earth-moving vehicles at the site that indicate crews were trying to clean it of radioactive traces.

Two of the diplomats said those traces could have come from what they said was the testing of a small neutron trigger used to set off a nuclear explosion. A third diplomat could not confirm that, but says any testing of a so-called neutron initiator at the site could only be in the context of trying to develop nuclear arms.

Iran faces growing international pressure over its nuclear program, which it insists is peaceful. Israel has hinted that it might resort to a pre-emptive military strike to stop Tehran's program.