The operator of the Fukushima Daiichi complex said radiation levels around the nuclear site rose fast immediately after the blast, the third there, but were still far from levels authorities say would cause large-scale radiation sickness.

Authorities are trying to prevent meltdowns in all three of the plant's nuclear reactors by flooding the chambers with sea water to cool them down.

Japan has asked the United States for more equipment to help cool the reactors, after a dangerous drop in cooling water levels that exposed fuel rods in the No. 2 reactor, where Tuesday's blast took place.

As concern about the crippling economic impact of the double disaster mounted, Japanese stocks plunged 7.2 percent to their lowest level in nearly two years, compounding a drop of 7.6 percent the day before.

Reuters