Japan's nuclear safety agency is reporting an emergency at a second reactor in the same complex where an explosion had occurred earlier.

The Nuclear and Industrial Safety Agency said early Sunday that the cooling system malfunctioned at Unit 3 of the Fukushima Dai-ichi nuclear plant. The agency said it was informed of the emergency by Tokyo Electric, the utility which runs the plant.

No further details of the troubles at Unit 3 were immediately available.

An explosion occurred at another reactor in the complex on Saturday, destroying the building housing the reactor and handing authorities an urgent complication amid rescue and relief efforts a day after Friday's earthquake and tsunami devastated Japan's northeastern coast.