The plant’s operator says dangerously high radiation levels recorded in water at one reactor raise the possibility its core has been damaged.
Prime Minister Naoto Kan said the situation was “very unpredictable”.
It is two weeks since the earthquake and tsunami hit Japan’s north-east. The official death toll has passed 10,000.
More than 17,440 people are missing.
Hundreds of thousands of people have been made homeless; an estimated 250,000 people are living in emergency shelters. Food, water and fuel are in short supply.
The Japanese government has put the rebuilding cost at $309bn (£191.8bn).
Meanwhile in a televised address, Prime Minister Kan said: “The current situation is still very unpredictable. We’re working to stop the situation from worsening. We need to continue to be extremely vigilant.”
He also thanked the workers, firefighters and Self-Defence Forces for “risking their lives” to try to cool the reactors at the Fukushima Daiichi plant.
Japan’s chief cabinet secretary Yukio Edano has said a rigorous inquiry is under way to establish the cause of a leak at the plant, after tests showed water in reactor 3 had radiation levels 10,000 times higher than normal.
Japan’s Nuclear Safety Agency has reiterated its position that it believes the reactor may have been damaged but it is going no further than that, says the BBC’s Mark Worthington in Tokyo.