One of the reactors at Japan's crippled nuclear power plant has been damaged more severely than originally thought, officials said, a serious setback for efforts to stabilise the radiation-leaking complex.

The data released on Thursday also showed that the water level in the core of Unit 1 at the Fukushima Daiichi plant is much lower than previously thought, leaving the portion of the fuel rods still intact fully exposed.

Other fuel has slumped to the bottom of the pressure vessel and is thought to be covered in water.

The findings also indicate a greater-than-expected leak in that vessel. Radioactive water pouring from troubled reactors has flooded around the complex, hindering work to bring the plant under control.

However, temperatures in the unit are still far below dangerous levels because the plant's operator, Tokyo Electric Power Co., continues to inject new water to keep the rods cool.

That radioactive water is apparently leaking into and through the larger containment vessel.