Plutonium has been detected in soil at the facility and highly radioactive water has leaked from a reactor building.

Officials say the priority remains injecting water to cool the fuel rods.

Mr Kan told parliament the situation at the quake-hit 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.

Meanwhile National Strategy Minister Koichiro Gemba said the government could consider temporarily nationalising Tepco, the company running the plant.

On Monday shares in the company dropped to their lowest level in three decades.

Chief Cabinet Secretary Yukio Edano, in another press briefing, described the situation at Fukushima as "very grave".

Workers are battling to restore power and restart the cooling systems at the stricken nuclear plant, which was hit by a powerful quake and subsequent tsunami over two weeks ago.

The twin disasters are now known to have killed 10,901 people, with more than 17,000 people still missing across a swathe of northern Japan.

"We are doing our utmost efforts to contain the damage," Mr Edano said.

But he said work to safely remove contaminated water was also a priority.

On Monday highly radioactive water was found for the first time outside one of the reactor buildings at Fukushima plant.

The leak in a tunnel linked to the No 2 reactor has raised fears of radioactive liquid seeping into the environment.

Plutonium - used in the fuel mix for one of the six reactors - has also been found in soil at the plant, but not at levels that threaten human health, officials say.