The chief of the U.S. Nuclear Regulatory Commission said Wednesday that all the water is gone from one of the spent fuel pools at Japan's most troubled nuclear plant, but Japanese officials denied it.

If NRC Chairman Gregory Jaczko is correct, this would mean there's nothing to stop the fuel rods from getting hotter and ultimately melting down. The outer shell of the rods could also ignite with enough force to propel the radioactive fuel inside over a wide area.

Jaczko did not say Wednesday how the information was obtained, but the NRC and U.S. Department of Energy both have experts on site at the Fukushima Dai-ichi complex of six reactors. He said the spent fuel pool of the complex's Unit 4 reactor has lost water.

Jaczko said officials believe radiation levels are extremely high, and that could affect workers' ability to stop temperatures from escalating.

Japan's nuclear safety agency and Tokyo Electric Power Co., which operates the complex, deny water is gone from the pool. Utility spokesman Hajime Motojuku said the "condition is stable" at Unit 4.