The Japan Meteorological Agency issued a "yellow" warning early on Saturday morning that a small tsunami would reach the Japanese coast at Fukushima, site of a wrecked nuclear power plant, at 1740 GMT.

There were no immediate reports of damage on land from the quake, classified as magnitude 6.8 by Japanese authorities, which struck some 230 miles off Japan's eastern coast. Japanese television images of harbors showed calm waters, said Reuters.

The Fukushima region is home to the nuclear reactor devastated in 2011's earthquake.

The U.S. Pacific Tsunami Warning Center issued a bulletin indicating that a devastating tsunami was unlikely for key U.S. coastal regions.

A yellow warning is issued when a tsunami is not expected to exceed a height of 1 metre (3 feet), far smaller than the wave that hit the plant in 2011.

A Reuters reporter tweeted that the plant's operator, Tepco, indicated no irregularities after the earthquake.