While reactors at the Kashiwazaki-Kariwa power plant automatically shut down when the 6.8-magnitude quake hit, it took nearly two hours to contain a blaze that began at one of the facility's electric transformers, the Kyodo news agency reported. The power company announced hours later that water containing radioactive material escaped from one reactor and some of it reached the Sea of Japan.
India Online reported officials said the leak was of "mildly radioactive water" and Sky News reported officials saying the leak produced "no significant changes" in nearby sea water.
The Mainichi Daily News said a 5.6-magnitude tremor hit Japan's Niigata Prefecture Monday afternoon, prompting experts to label it a possible aftershock of the initial quake.
With much of Japan's northwest coastal region losing power and water supplies, Japanese Prime Minister Shinzo Abe said returning such services was a national priority.
"I would like to make every possible effort to restore lifelines such as roads, gas and water, because the victims are in a tough situation," Abe said.