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

The new findings followed repairs to monitoring equipment, which also showed that the water level in the core of Unit 1 at the Fukushima Dai-ichi plant is much lower than previously thought, totally exposing remaining fuel rods and pointing to a greater-than-expected leak in the chamber.

However, temperatures there 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 water is apparently then leaking into and through the larger, beaker-shaped containment vessel and pooling up in other parts of the reactor building.

Nuclear Safety and Industrial Agency officials said the new data indicates that it is likely that partially melted fuel had fallen to the bottom of the pressurized core and possibly leached down into the outer containment vessel soon after the March 11 quake and tsunami that struck Japan's northeastern coast.

While officials said there was no danger that the chunks of fuel were still hot or that they could melt through the concrete base of the reactor, they acknowledged that the level of damage could complicate plans detailed in April to bring the plant to a cold shutdown within nine months.

Further examination of the pressure vessel was needed to ascertain the full extent of damage, they said.

"The situation (in the core) hasn't changed since (early in the crisis), and the fuel rods are being cooled by water continuously being injected into the core, although we still have to keep cooling them," NISA official Takashi Sakurai said.

The low level of water indicates that the core of Unit 1 had a bigger breach than expected, said TEPCO spokesman Junichi Matsumoto. Growing pressure inside Unit 1 led to a hydrogen explosion that damaged the outer building.

TEPCO workers have been struggling for two months to bring the plant under control.

On Wednesday, the plant operator detected a small new leak of radioactive water from an area near the Unit 3 reactor building into the sea. The plant has a total of six reactors, two of which have been totally shut down.