The International Atomic Energy Agency (IAEA) has accused Iran of furtively shopping on the international black market to acquire components for its nuclear programme.
In an interview on the BBC, Muhammad al-Baradei, the head of the United Nations nuclear watchdog said on Friday that Iran's nuclear programme had gone on far longer than the agency had realised.
Although he was not certain of the countries that made the equipment Iran had acquired on the black market, al-Baradei said he had a pretty good idea which ones they were.
"It could be one country, it could be more than one country," al-Baradei said.
"Iran told us they have got a lot of that stuff from the black market. It is through intermediaries. It is not directly from the country," he added.
Stopping just short of accusing Tehran of lying to the UN agency, al-Baradei said Iran had failed to give the IAEA a complete picture of its nuclear programme.
"They have not really been fully transparent in telling us in advance what was going on," he said.
'It could be one country, it could be more than one country'
The UN nuclear watchdog chief said he was not yet sure whether Tehran was also seeking to make nuclear weapons, a charge also levelled against Iran by the US.
"It might be, it might not be," al-Baradei said.
"I need to really get the Iranians to tell me the full, complete story," he said. "And I would like Iran to be more proactive, more transparent."
He said it would have been much easier to verify Iran's insistence its nuclear programme was for peaceful purposes had the IAEA been given a complete picture of its nuclear plans from the beginning.
"It would have been easier for us to complete our job if we knew what was going on as early as the mid 1980s," he explained.