The International Atomic Energy Agency says Iran has secretly manufactured enriched uranium and plutonium in violation of the non-proliferation treaty.
A confidential report by the IAEA says that Iran's nuclear plans go back almost two decades.
According to the 29-page report, Iran repeatedly violated international obligations, gave false or contradictory information on its nuclear ambitions and even has begun to separate plutonium.
Iran says its nuclear ambitions are entirely peaceful, but a senior western diplomat who did not want to be named said, plutonium is not necessary to switch on light bulbs. The diplomat said there is no evidence to suggest that Iran has stopped concealing the facts.
IAEA spokesman Mark Gwozdecky says scientists are looking into how weapons-grade uranium, or HEU, came to be present on equipment at facilities in Iran.
"We have enough information to pursue over the coming weeks and months an investigation, but obviously on the origin of the HEU [high-enriched uranium] issue the work in large part is outside of Iran; it's with other countries," he said.
Mr. Gowzdecky did not want to name any countries, but diplomats say Pakistan is a prime suspect supplier.
The IAEA board of governors meets next week to discuss the report and decide if the matter should go to the U.N. Security Council.
Diplomats say the United Nations has a range of options available, besides sanctions, such as issuing a statement supporting further investigations by the IAEA.