The operator of the crippled Fukushima Daiichi nuclear power plant says it has finished removing soil that may have been contaminated following last week's accidental leakage of untreated water.

The incident happened last Wednesday, when about 5.5 tons of water containing radioactive substances leaked into the environment from a water-purification system. The water contained radioactive substances far exceeding the level required for reporting to the government.

The operator, Tokyo Electric Power Company, said earlier that the outflow may have occurred during the cleaning of pipes installed on the filtering device.

Subsequently, TEPCO worked to remove the soil that the leaked water is believed to have seeped into. The utility said on Wednesday that it removed a total of 27 cubic meters of soil. It added that workers sealed the area with resin to prevent any potential spread of contamination.

The utility said the radiation level at the soil surface measured 19 millisieverts per hour when the work began. The level is now below 0.02 millisieverts per hour, the standard level before the leakage.

The leakage may have occurred due to workers possibly forgetting to shut the valves on the pipes, as 10 of the system's 16 valves had been left open at the time of the incident.

Criticism that the leakage may have resulted from possible dereliction of duty came at a meeting of Japan's Nuclear Regulation Authority on Wednesday.

The regulators plan to question the utility in detail at a hearing on Monday.