A strong 6.1-magnitude undersea earthquake rattled northeast Japan at around midnight on Wednesday, but there were neither immediate reports of damage nor tsunami alert.

The quake hit at 1502 GMT at a depth of 40 kilometres (25 miles), the US Geological Survey said, 120 kilometres from the city of Hakodate, on the northern Hokkaido island.

It was off the same coast but further north from the 9.0-magnitude earthquake in March last year that triggered a monster tsunami, leaving about 19,000 people dead or missing and crippling the Fukushima nuclear power plant.