Radioactive materials released from Japan's crippled Fukushima Daiichi nuclear power plant had spread around the entire northern hemisphere in the two weeks following the March 11 quake and tsunami disaster, a Vienna-based international nuclear watchdog said Thursday.

The Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization said minute traces of radioactive substances spread around the hemisphere by around March 25 after moving across the Pacific Ocean and other places. It said the amounts of such substances were far below levels that could affect human health.