TOKYO—The Japanese government Friday published a report on the discharge of more than 10,000 metric tons of low-level radioactive water from the quake-hit Fukushima Daiichi nuclear complex, in a bid to allay concerns among neighboring countries that it was spreading contamination into the ocean.

Plant operator Tokyo Electric Power Co. released a total of 10,393 tons of radioactive water April 4 to 10, according to the report published Friday evening local time by Japan's Nuclear and Industrial Safety Agency, made up of 1,323 tons of groundwater and 9,070 tons of seawater.

The agency said the analysis showed that the water was only lightly contaminated. Most of the radiation was believed to have come from a series of hydrogen explosions that hit the plant in the first week, which was then brought down in subsequent rainfall, it said.

The government estimated the total amount of radiation contained in the released water at 150 billion becquerels—exceeding the legal limits by about 100 times—depending on the sample taken, according to Hiroki Ishigaki, an official with the agency. The data showed that iodine-131, which has a short half-life of eight days, was the most commonly found isotope, with the longer-lived cesium-134 and cesium-137 at lower but still elevated levels.

The report follows criticism, especially from South Korea and China, that Japan hadn't provided advance warning of its plans and that the release could pose environmental threats.