A Japanese nuclear safety panel says radiation levels are 1000 times above normal in a reactor control room after a huge quake damaged a plant's cooling system. People living nearby have been told to evacuate.
Public broadcaster NHK, quoting nuclear safety officials, said there was "no immediate health hazard" to nearby residents from a possible minute leakage, and people were urged to evacuate the area calmly.
The elevated reading was taken in the control room of the No.1 reactor of the Fukushima No.1 nuclear power plant, said the Nuclear and Industrial Safety Agency of the Ministry of Economy, Trade and Industry.
Kyodo News said that the discovery suggested radioactive steam could spread around the facility, which is located about 250km northeast of Tokyo and operated by Tokyo Electric Power.
Tokyo Electric said it had lost its ability to control pressure in some of the reactors of a second nuclear power plant at its quake-hit Fukushima facility, according to Reuters.
Pressure is stable inside the reactors but rising in the containment vessels, a spokesman said, although he did not know if there would be a need to release pressure at the plant at this point, which would involve a release of radiation.
Prime Minister Naoto Kan said early on Saturday that residents living within 10km of the plant must evacuate, before stepping onto a helicopter to head for the area.
His government on Friday declared an atomic emergency and told thousands of residents living within a smaller radius of three kilometres, to evacuate, warning a small amount of radiation could be released.
Separately, the US military did not provide any coolant for a Japanese nuclear plant affected by a massive earthquake on Friday, US officials said.
US Secretary of State Hillary Clinton earlier had said that US Air Force "assets" had delivered "some really important coolant" to a Japanese nuclear power plant.
One US official said he believed Clinton was told Japan had requested the material, that the United States had agreed to provide it, and that an operation to do so was under way.
Ultimately, however, Japan did not need assistance from the United States but Clinton did not appear to have been updated before she made her public remarks.
"We understand that ultimately the Japanese government handled the situation on its own," said another US official, who spoke on condition of anonymity.