AN explosion at a nuclear power station this morning destroyed a building housing the reactor - but a radiation leak was decreasing despite fears of a meltdown from damage caused by a powerful earthquake and tsunami, officials said.

Government spokesman Yukio Edano said the explosion destroyed the exterior walls of the building where the reactor is placed, but not the actual metal housing enveloping the reactor.

That was welcome news for a country suffering from Friday's double disaster that pulverised the north-eastern coast, leaving at least 574 people dead by official count.

The scale of destruction was not yet known, but there were grim signs that the death toll could soar.

One report said four whole trains had disappeared and still not been located.

Local media reports said at least 1,300 people may have been killed.

Edano said the radiation around the Fukushima Dai-ichi plant had not risen after the blast, but had in fact decreased. He did not say why that was so.

The pressure in the reactor was also decreasing after the blast, he said.

The explosion was preceded by puffs of white smoke that gathered intensity until it became a huge cloud enveloping the entire facility, in Fukushima, 20 miles from Iwaki.

After the explosion, the walls of the building crumbled, leaving only a skeletal metal frame.

Tokyo Power Electric Co, which runs the Fukushima Dai-ichi plant, said four workers suffered fractures and bruises and were being treated at a hospital.

''We have confirmed that the walls of this building were what exploded, and it was not the reactor's container that exploded,'' said Edano.

The trouble began at the plant's Unit 1 after the massive 8.9-magnitude earthquake and the tsunami it spawned knocked out power there, depriving it of its cooling system.