Japanese authorities began evacuating more than 200,000 people Saturday from areas around two nuclear power plants after an explosion at one of them damaged a building housing an aging U.S.-supplied reactor.

The unit, built 40 years ago by General Electric, is one of as many as seven reactors imperiled by the earthquake and subsequent disruptions in the power supply that the reactors use for cooling systems.

One nuclear worker was killed, at least six were injured and two were missing at the plants in the aftermath of Friday's 8.9 magnitude earthquake and the blast Saturday afternoon, according to the Tokyo Electric Power Co., which operates the Fukushima I and II nuclear power stations.

The company said it was taking steps to relieve pressure that has built up in seven of the plants' 10 reactor containment vessels. It said this would be done through a "partial discharge of air containing radioactive materials." The three other reactors - at the Fukushima I plant - had already been shut down for a regular inspection when the earthquake struck.

The International Atomic Energy Agency (IAEA) said Saturday that Japanese authorities informed it that the explosion at the Unit 1 reactor of the Fukushima I plant "occurred outside the primary containment vessel," a steel structure that houses the reactor.

Japan's chief government spokesman, Yukio Edano, told reporters that the blast occurred when vapor from the steel container turned into hydrogen and mixed with outside oxygen. Edano said the explosion blew off the roof and walls of the building around the containment vessel but did no serious damage to the vessel itself.

The explosion sent white smoke billowing into the air and prompting Japanese officials initially to warn people in the vicinity to cover their mouths and stay indoors.

Authorities ordered an estimated 170,000 people to evacuate from a 20-kilometer (12.4-mile) radius around the Fukushima I plant and about 30,000 people to leave a 10-kilometer (6.2-mile) radius around the Fukushima II plant. The two plants, also known respectively as Fukushima Daiichi and Fukushima Daini, are about seven miles away from each other in Fukushima Prefecture.

To limit damage to the reactor core at the site of the blast, plant officials began injecting sea water mixed with boron into the primary containment vessel at 8:20 p.m. Saturday local time.

The IAEA said Japan classified the event at Unit 1 as a Level 4 "accident with local consequences" on the International and Radiological Event Scale.

Tokyo Electric Power said four workers were injured in the explosion, one was exposed to radiation and two were missing. It said a worker who was trapped in the crane operating console of the exhaust stack at the Fukushima Daini plant died of his injuries and that another worker there was slightly injured.

Japanese government officials suggested Saturday evening that a widespread radioactive leak could be avoided.