A fresh explosion rocked the crippled Fukushima nuclear plant, raising radiation in the area to dangerous levels, the Japanese government said Tuesday.

The explosion was heard at the No. 2 reactor of Fukushima No.1 nuclear power plant at 6:10 a.m. (2110 GMT on Monday) and a fire later broke out at the No. 4 reactor, causing more radiation near the complex.

Prime Minister Naoto Kan warned of further leaks and told people living within a radius of 30 km of the troubled plant to stay indoors.

Tokyo Electric Power Co. (TEPCO), owner and operator of the Fukushima complex, said it was evacuating all staff except those working to cool the reactor, signalling a deterioration in the situation.

The latest blasts come after previous explosions at its No. 1 and No. 3 reactors, caused by damage from Friday's earthquake and tsunami, which devastated much of north-west Japan.

According to Japan's Nuclear and Industrial Safety Agency, radiation had fallen to 882 micro sievert after briefly rising as high as 965.5 micro sievert, both well above the safety limit of 500.

The agency also said the suppression pool of the No. 2 reactor might have been damaged.

Before the blast, workers had resumed pumping seawater into the reactor unit to cool it after a steam vent of the pressure container was opened.

The nuclear plant shut down automatically during the massive earthquake but its cooling systems were knocked out, raising fears of a major radiation leak and possible meltdown.