"Transparency is an absolute key if they want to clear their name and for us to be able to conclude that the program is completely for peaceful purposes," ElBaradei, the head of the International Atomic Energy Agency (IAEA), said on CNN's "Late Edition."

Iran has been cooperating fully with the IAEA, but the discovery of the extent of its program has created skepticism, he said.

ElBaradei said he hoped to touchdown in Iran "in the next couple of weeks."

Iran's government has said IAEA inspectors can return to the country on March 27, after originally postponing an early March mission in order to protest against the agency's tough resolution against the Islamic Republic.

"Iran had agreed to fully suspend its enrichment program as a confidence-building measure, so we have to acknowledge we have made a good headway along our effort to make sure that (the) Iran program is completely for peaceful purpose," the United Nation's inspector said.

"However, in the process we have discovered ... that this is a sophisticated program, it's an extensive program and it's a program that has been undeclared for over 15 years," he said.

"And in that context, as you understand, there's still a lot of skepticism that something might still be hidden," he added.

ElBaradei, who took part in UN weapons inspections inside Iraq prior to the US-led invasion, said inspections had dismantled Iraq's nuclear program in

"We learned from Iraq that an inspection takes time, that we should be patient, that an inspection can, in fact, work," he said.

But Iraq should have been transparent with inspectors, he added.

"But one of the lessons that, if a country really wants to show to the world that its programs are peaceful, weapons of mass destruction program are peaceful, they ought to be transparent, they ought to take a proactive approach."

WAR.WIRE