Iran said on Monday that all its nuclear activities are under IAEA watch.

Iran said on Monday that all its nuclear activities are under IAEA watch, including those at a new uranium enrichment bunker at Fordo that one Western diplomat said was already up and running.

"All nuclear activities, notably uranium enrichment at Natanz and Fordo, are under the supervision of the International Atomic Energy Agency," the Iranian envoy to the International Atomic Energy Agency, Ali Asghar Soltanieh, was quoted as saying by Iran's Arabic Al-Alam broadcaster.

While nuclear energy plants need fuel enriched to 3.5 percent, Iran says the 20-percent enriched uranium is needed for its Tehran research reactor to make isotopes to treat cancers.

Fordo, a reinforced facility sunk deep under a mountain 150 kilometers (90 miles) southwest of Tehran, is designed to be a difficult if not impossible target to bomb.

Natanz, which is also a hardened target though more vulnerable than Fordo, is already operating with some 8,000 centrifuges working to enrich uranium.

The United Nations has passed four rounds of sanctions against Iran for its refusal to halt uranium enrichment, and Western powers are looking to impose even tougher punitive measures.

Iran's supreme leader Ayatollah Ali Khamenei said that Tehran will not falter in the face of the sanctions, in comments broadcast Monday on state television.

"The firm decision of the Islamic Republic is to resist the pressures" of Western governments, Sayyed Khamenei said.

"While the Iranian people have travelled the road to success and see the signs of new victories to come, the (Western) oppressor is trying to frighten the Iranian people and officials by brandishing the threat of sanctions," Grand Ayatollah added.