Tokyo (CNN) -- Highly radioactive water from Japan's crippled Fukushima Daiichi nuclear power plant is leaking into the Pacific Ocean from a cracked concrete sump near the No. 2 reactor, an official with the plant's owner said Saturday.

Water from the two-meter deep, concrete-lined basin could be seen escaping into the sea through a roughly 20-cm (8-inch) crack, an official the Tokyo Electric Power Company told reporters Saturday afternoon. But the company could not explain how the water was getting into the sump, which is a pit in which liquid collects.

Radiation levels in the pit have been measured over 1,000 millisieverts per hour, or more than 330 times the dose an average resident of an industrialized country receives in a year. Utility company officials said Saturday that the plan was to to fill the sump with concrete in order to stop the leakage.

Authorities have been working feverishly to explain a sharp spike in contamination in seawater measured just off the plant, which is located 240 kilometers (150 miles) north of Tokyo. The company has retracted some alarmingly high readings in recent days, and Japanese regulators said Saturday that new figures were being reviewed to ensure their accuracy.