Tokyo: The operator of Japan's stricken Fukushima nuclear plant reported new problems on Thursday with a water leak from a reactor vessel and another spill of contaminated water into the ocean.

The update by Tokyo Electric Power Company (TEPCO) came as emergency crews have been battling to bring the tsunami-hit and radiation-leaking plant into stable ‘cold shutdown’ sometime between October and January.

Pressure and temperature build-ups have triggered a series of explosions, and workers have since doused the reactors and fuel rod pools with water to stop them from overheating and releasing greater amounts of radiation.

TEPCO on Thursday said new measurements taken this week, after workers in protective suits fixed gauges in the badly-hit reactor one building, indicated that water pumped into the pressure vessel had quickly leaked out.

The water level inside had fallen below the bottom of the four-metre (12 foot) long fuel rods, suggesting they had been exposed to the air, increasing the risk of a dangerous full meltdown.

However, the vessel's relatively low outside temperature of 100-120 degrees Celsius (212-248 degrees Fahrenheit) indicated that the rods had dropped to the bottom of the vessel and were under water, TEPCO said.

Anti-nuclear and environmental activist group Greenpeace said the news showed that at least some water level readings were entirely wrong and that "the situation is clearly far more serious than previously reported".