SALT LAKE CITY — Radioactivity from Japan's nuclear power plant disaster last year has been detected as far away as almost 643 kilometres offshore in the Pacific Ocean.

Scientists say ocean water showed readings of up to 1,000 times prior levels of the substance cesium-137 from the Fukushima nuclear power plant.

But they say the new readings are far below the levels that are generally considered harmful, either to marine animals or people who eat seafood.

The results are from water samples taken in June, about three months after the disaster caused by a powerful earthquake and the tsunami that followed.

The findings were reported Tuesday at a scientific meeting in the U.S.