TOKYO — The stricken nuclear power plant at Fukushima has probably been leaking contaminated water into the ocean for two years, ever since an earthquake and tsunami badly damaged the plant, Japan’s chief nuclear regulator said on Wednesday.

In unusually candid comments, Shunichi Tanaka, the head of the Nuclear Regulation Authority, also said that neither his staff nor the plant’s operator knew exactly where the leaks were coming from, or how to stop them.

The operator, Tokyo Electric Power, has reported spikes in the amounts of radioactive cesium, tritium and strontium detected in groundwater at the plant, adding urgency to the task of sealing any leaks. Radioactive cesium and strontium, especially, are known to raise risks of cancer in humans.

Mr. Tanaka’s comments bring into sharp relief the precariousness of the cleanup at the Fukushima Daiichi Nuclear Power Plant, where core meltdowns occurred at three of the six reactors. A critical problem has been the groundwater that has been pouring into the basements of the damaged reactor buildings and becoming contaminated. Workers have been pumping the water out to be stored in dozens of tanks at the plant, but have not stopped the inflow.