Western allies claim Iran is using a civilian nuclear program to hide a more controversial weapons program. Tehran, which sits on some of the largest oil and gas deposits in the world, maintains its program is for peaceful purposes.
Yukiya Amano, the head of the International Atomic Energy Agency, told German news magazine Der Spiegel that he was frustrated by Iran's lack of transparency.
"We still know too little about Tehran's nuclear activities," he was quoted as saying. "And we still don't have answers to all of our questions, which is why we make it clear that Tehran has to cooperate with us more effectively."
He said he deplored the fact that Iran wasn't working more closely with the IAEA, which he said could develop trust with the international community. Iran, however, was still ignoring calls to halt its uranium enrichment activity.
Amano said he wasn't ready to state that Iran was moving closer to a nuclear weapon, but noted there was a "credibility gap" when it came to reporting its nuclear work to the international community.