Tokyo Electric Power Co. told Kyodo News a worker at the power plant spotted the fire on the fourth floor of the reactor building about 5:45 a.m. It was believed to be the same area where an explosion and fire occurred Tuesday, the Japanese news agency said.
Tokyo Electric said the blaze had been reported to the fire department and local governments.
High radiation levels were detected Tuesday at the power plant, officials said.
The fire, which Chief Cabinet Secretary Yukio Edano said was likely caused by another hydrogen explosion at the facility, was extinguished, Kyodo reported.
Edano told reporters the level of radiation "may pose health risks to humans," the Yomiuri Shimbun reported.
Experts said they feared a massive amount of radioactive material leaked from reactors since a magnitude-9 earthquake and tsunami struck Japan Friday.
Andre-Claude Lacoste, president of France's nuclear safety authority, said the disaster is rated a Level 6, just below the 1986 explosion at a nuclear plant in Chernobyl, Ukraine, CNN reported. That explosion contaminated a wide area that included parts of Ukraine, Belarus and Russia with nuclear fallout. At least 30 people died at the time of the incident but thousands more -- one estimate is as high as 985,000 -- are believed to have died of illnesses triggered by radiation exposure in the years since.
Minute amounts of radioactive material were detected in Ibaraki and Tochigi prefectures and Tokyo Tuesday, local governments said.
Prime Minister Naoto Kan urged the approximately 136,000 residents within a 12- to 18-mile radius of the Fukushima power plant to remain indoors, Yomiuri Shimbun said.
Tokyo Electric and other sources said high levels of radiation were detected at multiple sites near the plant.
"The levels are completely different from the … figures we had announced previously," Edano said. "These figures may cause health damage."
Officials estimated radiation levels had reached about 40 rems, The Wall Street Journal reported. A single dose of 25 rems can cause temporary sterility in men. One hundred rems can cause radiation sickness and 500 rems likely will cause death.
Kan urged calm but warned that there was "a very high risk" of more leakage. The prevailing winds had so far swept most of the radioactive plume over the Pacific Ocean instead of populated areas, officials said.
"We are on the brink. We are now facing the worst-case scenario," Hiroaki Koide, a senior reactor engineering specialist at the Research Reactor Institute of Kyoto University, told The New York Times.