The investigation tends "on balance to support Iran's statement about the foreign origin of most of the observed highly enriched uranium contamination," the International Atomic Energy Agency said in a confidential report seen by

But "it is still not possible at this time, however, to establish a definitive conclusion with respect to all of the contamination," the report said, referring both to anomalies in the high enriched uranium data and findings for low-enriched uranium which is not weapons-grade.

"This underscores the importance of additional work on the scope and chronology of Iran's P-1 and P-2 centrifuge programs, which could greatly contribute to the resolution of the remaining contamination issues," the IAEA said.

Uranium is enriched by cascades of centrifuges into what can be fuel for nuclear power reactors or, in highly enriched form, the raw material for atom bombs.