A suspected breach in the reactor at the stricken Fukushima nuclear plant could mean more serious radioactive contamination, Japanese officials have warned.
Prime minister Naoto Kan called the country's fight to stabilise the plant "very grave and serious".
The sombre leader sounded a pessimistic note at a briefing hours after nuclear safety officials announced what could be a major setback in the urgent mission to stop the plant from leaking radiation.
"The situation today at the Fukushima Dai-ichi power plant is still very grave and serious. We must remain vigilant," Mr Kan said. "We are not in a position where we can be optimistic. We must treat every development with the utmost care."
His warning came two weeks after a devastating earthquake and tsunami disabled the plant.
The escalation in the nuclear plant crisis came as the death toll from the quake and tsunami passed the grim milestone of 10,000.
Across the battered north-east coast, hundreds of thousands of people whose homes were destroyed still have no power, no hot meals and, in many cases, no showers for 14 days.
The uncertain situation halted work at the nuclear complex, where dozens had been trying to stop the overheated plant from leaking dangerous radiation. The plant has leaked some low levels of radiation, but a breach could mean a much larger release of contaminants.
The government has already ordered people living 12 miles from the plant to evacuate because of radiation leaks.
Government spokesman Yukio Edano reaffirmed that people farther away from that limit were safe, but that those within 20 miles of the plant should stay indoors. He added that since supplies are not being delivered to the area fast enough, it may be better for residents to voluntarily evacuate to places with better facilities.