This is so, the CIA claims, because Tehran could be using legitimate fuel production to cover up its weapons programme.
The assessment, contained in a report to Congress made public over the weekend, coincided with Iran's formal assurances to the International Atomic Energy Agency that it will accept surprise UN inspections of its nuclear facilities and suspend its uranium enrichment programme.
The two promises had been requested by the IAEA ahead of its 20 November meeting to assess Iran's compliance with the nuclear Non-Proliferation Treaty.
But the CIA said a secret nuclear facility at Natanz whose existence had been disclosed by members of the Iranian opposition in August 2002 could be cause for concern.
About 160 new centrifuges for enriching uranium have subsequently been discovered at that complex, located between the cities of Isfahan and Kashan in central Iran, according to US and UN officials.
"Even with intrusive IAEA safeguards inspections at Natanz, there is a serious risk that Iran could use its enrichment technology in covert activities," warned the CIA.
It added that the uranium centrifuges discovered at Natanz were "of specific proliferation concern" because they are capable of enriching uranium for use in nuclear weapons.
"Although Iran claims that its nascent enrichment plant is to produce fuel for the Russian-assisted construction projects at Bushehr and other possible future power reactors, we remain concerned that Iran is developing enrichment technology to produce fissile material for nuclear weapons under the cover of legitimate fuel cycle activities," the CIA said.
The report insisted that Iran appears to be trying to produce both known materials for making nuclear warheads - highly enriched uranium and low burn-up plutonium.
According to the CIA, commercial satellite imagery showed that Iran was trying to bury the Natanz enrichment facility in the ground, presumably to hide it or harden it against military attack.
"We also suspect that Tehran is interested in acquiring fissile material and technology from foreign suppliers to support its overall nuclear weapons programme," the report said.