The head of Japan's nuclear regulatory body has said the instruction he gave to his body's secretariat to review guidelines for responding to nuclear accidents was not prompted by the earthquake that struck central Japan on New Year's Day.

Guidelines set by the Nuclear Regulation Authority call for residents outside a 5-kilometer radius of a nuclear plant but within 30 kilometers to shelter indoors in the event of a serious accident.

But the January 1 earthquake cut off communities and caused widespread structural damage on the Noto Peninsula, making it difficult for people to remain indoors.

Authority Chairman Yamanaka Shinsuke told reporters on Wednesday that many questions about indoor sheltering were raised at a meeting on Saturday with officials from municipalities around the Onagawa nuclear plant in Miyagi Prefecture.

He stressed that these questions prompted him to give the instruction to the authority's secretariat.

Participants in Saturday's meeting had asked for better nuclear disaster preparedness, including plans for evacuation and indoor sheltering, noting that the Noto quake destroyed many homes and made a number of roads impassable.

Reporters asked Yamanaka how residents near a nuclear plant should respond in a huge earthquake, when they could be deprived of means of evacuation or sheltering facilities.

The chairman said the guidelines stipulate that measures against natural disasters are fundamental.

He also said regional disaster preparedness plans are in place where nuclear plants are in operation, and related facilities are being built.

He added that evacuations by ship and aircraft are being planned for areas that could become cut off, and suggested that means of evacuation and facilities for indoor sheltering are being secured.

He said the authority's discussions will focus on when indoor sheltering should begin and how long it should continue, and that it will take several months to reach a conclusion.