VIENNA -- Radioactive fallout from Japan's crippled nuclear plant has reached Southern California but the first readings are far below levels that could pose a health hazard, a diplomat said Friday.
The diplomat, who has access to radiation tracking by the U.N.'s Comprehensive Test Ban Treaty Organization, cited readings from a California-based measuring station of the group.
Initial readings are "about a billion times beneath levels that would be health threatening," the diplomat told The Associated Press, speaking on condition of anonymity because the CTBTO does not make its findings public.
The organization forecast earlier this week that some radioactivity would reach Southern California by Friday. A CTBTO graphic obtained Thursday by the AP showed a moving plume reaching the U.S. mainland after racing across the Pacific and swiping the Aleutian Islands.
The diplomat's comments backed up expectations by IAEA officials and independent experts that radiation levels -- which are relatively low outside of the immediate vicinity of the Japanese plant -- would dissipate so strongly by the time it reached the U.S. coastline that it would pose no health risk whatsoever to residents.
While set up to monitor atmospheric nuclear testing, the CTBTO's worldwide network of stations can detect earthquakes, tsunamis and fallout from nuclear accidents such as the disaster on Japan's northeastern coast that was set off by a massive earthquake and a devastating tsunami a week ago.
Since then, emergency crews have been trying to restore the Fukushima Dai-ichi nuclear plant's cooling system and prevent overheated fuel rods from releasing massive doses of radioactivity.
Japanese officials on Friday reclassified the rating of the accident at the plant from Level 4 to Level 5 on a seven-level international scale, putting it on a par with the 1979 Three Mile Island accident. The International Nuclear Event Scale defines a Level 4 incident as having local consequences and a Level 5 as having wider consequences.
Nuclear experts have been saying for days that Japan was underplaying the severity of the nuclear crisis.
Yukiya Amano, the head of the Vienna-based International Atomic Energy Organization, left for Tokyo on Thursday to assess the situation. He plans to return on the weekend and to brief the IAEA's 35-nation board in an emergency session Monday.
In Geneva, the World Health Organization said Tokyo's radiation levels are increasing but are still not a health risk, and the group sees no reason to ban travel to Japan because of its nuclear crisis.
WHO spokesman Gregory Hartl said Friday the organization "is not advising travel restrictions to Japan" outside the 18.6-mile exclusion zone around the Fukushima Dai-ichi nuclear complex.
Hartl said includes Tokyo where "radiation levels have increased very slightly, but are still well below the absolute levels of radiation where it would be considered a public health risk."
He also said "in general travelers returning from Japan do not represent a health hazard."