A powerful earthquake has struck off the coast of northern Japan, triggering a tsunami alert, the Japanese Meteorological Agency said.
The agency said the magnitude 7.2 quake struck just before 2am off the coast of Hokkaido, near the coastal city of Aomori, with an epicentre about 50 kilometres below the sea surface.
It issued an alert in the region for a tsunami of up to 3 metres.
A tsunami warning is displayed on a television in Yokohama, near Tokyo. (AP)
Nuclear power plants in the region were conducting safety checks, public broadcaster NHK reported.
