Nuclear experts responsible for monitoring Iran's program said they've discovered that enough uranium, which if enriched could make up to six bombs, was no longer at the Isfahan nuclear production facility, The Daily Telegraph reported Friday.

Spy satellites identified suspicious sites that Iran hasn't declared to nuclear inspectors, the British newspaper said.

The Isfahan facility, where raw uranium is enriched so it can be used for either nuclear power or atomic weapons, is subject to International Atomic Energy Agency supervision. Iran, however, has given IAEA inspectors access only to the final stage of the production process, where the uranium in a gaseous state is stored.

Nuclear experts said they've determined that 50-60 tons of unprocessed uranium is missing, the newspaper said.

"The inspectors only have limited access at Isfahan and it looks as though Iranian officials have removed significant quantities ... at a stage in the process that is not being monitored," a nuclear official told the Telegraph. "If Iran's nuclear intentions are peaceful, then why are they doing this?"

IAEA officials told The Daily Telegraph they suspect the missing uranium is being stored in installations spotted by U.S. spy satellites.

The Iranians will be asked to account for missing enriched uranium when the IAEA's board of governors meets in Vienna this month, officials said.