Tokyo Electric Power (Tepco) made the announcement three weeks after failing to bring reactors 1 - 4 under control. Locals would be consulted on reactors 5 and 6, which were shut down safely.
Harmful levels of radioactivity have been detected in the area.
More than 11,000 people are known to have been killed by the devastating 11 March earthquake and tsunami.
Emperor Akihito visited a centre for earthquake and tsunami victims in the Tokyo area on Wednesday.
The emperor's visit "gives me strength" said one of the evacuees.
"Amid all the destruction, it is a safe haven”
Japanese experts are considering whether to cover the reactor buildings at the Fukushima Daiichi plant with a special material, to stop the spread of radioactive substances, Chief Cabinet Secretary Yukio Edano says.
On Wednesday, the government ordered nuclear power plant operators to start implementing new safety measures immediately.
The steps - to be completed by the end of April - include preparing back-up power in case of loss of power supply.
Fire trucks will be on standby to intervene and ensure cooling systems for both reactors and pools of used fuel are maintained. (read more)