Emergencies have been declared at two nuclear power stations in Japan after the worst natural disaster in the country's history.
Radioactive steam has been released to reduce rising pressure in the plants and the Japanese prime minister has ordered thousands of people living within six miles to leave the area.
The 8.9 magnitude quake has left at least a 1,000 people dead and triggered a tsunami with waves that devastated parts of Japan's north-eastern coast and set off warnings as far away as California and Chile.
The nuclear plants are both at Tokyo Electric Power Company's Fukushima facility, situated 150 miles north of Tokyo.
Pressure at one - the Daiichi reactor - may have risen to 2.1 times the designed capacity, the country's trade ministry said.
Media reports also said the radiation level was rising in the turbine building.
Radiation levels outside the main gate of the plant were eight times normal levels.
Tepco later said it had released a small amount of radioactive vapour into the atmosphere to reduce the pressure and said there were no health risks.
Chief cabinet secretary Yukio Edano said: "Residents are safe... we want people to be calm."
Tepco said it had lost ability to control pressure in some of the reactors in both the Daiichi and Daini plants. Pressure was stable inside the reactors of the Daini but rising in the containment vessels, a spokesman said.
Japan informed the International Atomic Energy Agency (IAEA) that the quake and tsunami cut the supply of off-site power to the plant and diesel generators intended to provide back-up electricity to the cooling system.
If power is lost, the options to cool the core are limited. If the core overheats, then the fuel would become damaged and a molten mass could melt through the reactor vessel, releasing a large amount of radioactivity into the containment building surrounding the vessel.
It is unclear if the quake has undermined the containment building which might allow radioactivity to leak out.
Power supply systems that would provide emergency electricity for the plant were being put in place, the World Nuclear Association said.
The reactors shut down due to the earthquake account for 18% of Japan's nuclear power generating capacity.