Reporting from Tokyo and Beijing —

Fears of a radiation leak intensified Saturday after an explosion at a nuclear power plant in northeastern Japan that had been damaged a day earlier by a tsunami and earthquake.

The walls of a building at the Fukushima power plant were blown off in the blast, leaving only a skeletal frame. Officials said four workers at the site received non-life-threatening injuries.

At least one reactor at the plant was already showing signs of a partial meltdown. Friday’s magnitude 8.9 earthquake had prevented the plant 150 miles north of Tokyo from powering its water cooling system. . .