In another development on Friday, the Japanese government said it would help people who wish to leave the area around the crippled plant, a sign that efforts to reassure frightened residents have failed to persuade people to stay.

There is no evacuation order for a zone from 12 to 19 miles around the nuclear plant, the chief cabinet secretary, Yukio Edano, said in a televised news conference, but officials recognize that some people may wish to leave and it will assist them. The government has ordered all people living within about 12 miles of the plant to evacuate, but has only advised those in the outer zone to stay indoors.

Image A health warning prompted a run on bottled water in Tokyo. Credit... The New York Times

That advice has not changed, Mr. Edano said. The United States has recommended that its citizens stay at least 50 miles away from the plant.