Vienna - Japan's radioactive fallout has reached Southern California but first readings are "about a billion times beneath levels that would be health threatening," a diplomat with access to United Nations' radiation tracking said Friday.

The diplomat, who asked for anonymity Friday because the Comprehensive Test Ban Treaty Organization does not make its data public, cited readings Friday from one of the U.N.'s California-based measuring station.

IAEA officials and independent experts have emphasized that the radiation level was already low outside of the immediate vicinity of the crippled reactor.

They said it would dissipate so strongly by the time it reached the U.S. coastlines that it would pose no health risk whatsoever to residents there.

Any detectable radiation on Friday "could be coming from your own reactors in California," said physics Prof. Paddy Regan at the University of Surrey at Guildford in Britain.

A senior IAEA official who asked for anonymity because he was not authorized to speak to media, noted that even in Tokyo, "radiation was at background levels."