The new blaze began at in northeastern corner of Reactor 4 of the Fukushima Daiichi plant, a power official said.
The plant has already been hit by four explosions, triggering radiation leaks
and sparking health concerns.
More than 3,000 have been confirmed dead and thousands are missing following Friday's earthquake and tsunami in northeast Japan. The disaster has damaged the nuclear plant's cooling functions.
Officials have warned people within 20-30km of the site to either leave the area or stay indoors, the BBC reported.