Japan's crippled Fukushima Daiichi nuclear power plant is facing a delay in stabilising due to a reactor leaking water.
The water level in the pressure vessel containing uranium fuel rods in the No. 1 reactor has dropped about 5 metres below the targeted level to cover fuel under normal operating conditions.
Tokyo Electric Power Co (TEPCO) discovered the leak after repairing a gauge in the reactor earlier this week.
"There must be a large leak," Junichi Matsumoto, a TEPCO general manager said.
"The fuel pellets likely melted and fell, and in the process may have damaged the pressure vessel itself and created a hole."
It is the latest setback for the plant which has been at the centre of the worst nuclear disaster since Chernobyl after being damaged by a massive earthquake and tsunami.
Nuclear workers are are pumping water at four reactors to cool the melted uranium fuel and bring the rods to a "cold shutdown" state by January.
As the surface temperature of the pressure vessel has been holding steady between 100 and 120 degrees Celsius, Matsumoto said the effort was working and would continue.
TEPCO is considering increasing the amount of water it is injecting to overcome the leak and raise the level of water covering the fuel, at the risk of allowing more radioactive water to leak out of the facility.
Nearly 10,400 tonnes of water has been pumped into the reactor so far, but it is unclear where the leaked water has been going.
The high radiation makes it difficult for workers to check the site, Matsumoto said.
TEPCO announced a timetable last month for addressing the crisis, saying it aimed to cool reactors to a stable level and reduce the leakage of radiation within the first three months, then bring the reactors to a cold shutdown in another three to six months.
Officials had planned to use the same set of steps to stabilise reactors No. 2 and No. 3 that are under way at No. 1, which workers re-entered last week for the first time since the earthquake.
However it is likely that the pressure vessels in the other two reactors could be leaking as well if fuel rods had collapsed and melted after the earthquake and tsunami, Matsumoto said.
"It is necessary to make a reassessment of the condition of the nuclear reactor," Chief Cabinet Secretary Yukio Edano said.
The battle to bring Fukushima under control has been complicated by repeated leaks of radioactive water at the site.
TEPCO sealed a fresh leak of contaminated water found near the No. 3 reactor that may have seeped into the Pacific Ocean from the coastal plant.
A previous ocean leak sparked international concern about the impact of the disaster on the environment.