Japanese Prime Minister Naoto Kan says his government is in a state of maximum alert over the crippled Fukushima nuclear plant.
Plutonium has been detected in soil at the facility - seriously damaged in the 11 March earthquake and tsunami - and highly radioactive water has leaked from a reactor building.
The leak has raised fears of radioactive liquid seeping out. Officials say, however, that the level of plutonium does not threaten human health. They say the priority remains injecting water to cool the overheated fuel rods.
Mr Kan told Parliament the situation at the plant "continues to be unpredictable".
The government will "tackle the problem while in a state of maximum alert", he said, adding that he was seeking advice on whether to extend the evacuation zone around the plant.
In another news briefing reported by the BBC, Chief Cabinet Secretary Yukio Edano described the situation at Fukushima as "very grave".
Saying "We are doing our utmost to contain the damage," Mr Edano urged people evacuated from their homes near the plant not to return yet.
Officials in China, South Korea, Vietnam and the United States say they have recorded traces of radioactive material in the air, but nothing so great as to constitute a public health hazard.
The earthquake and tsunami are now known to have killed 10,901 people, with more than 17,000 people still missing across a swathe of northern Japan.