A satellite image of the Fukushima Nuclear Reactor following concerns over a build-up of radioactive ground water.

The operator of Japan's wrecked Fukushima nuclear plant said on Saturday there was no damage or spike in radiation levels at the station after a large earthquake struck in the ocean east of Japan, triggering a small tsunami.

There were no immediate reports of damage on land from the quake, classified as magnitude 7.1 by the Japan Meteorological Agency, which struck about 370 km (230 miles) out to sea. Earlier the agency said the quake had a magnitude of 6.8.

Japanese television said a 30 cm (1 foot) tsunami had reached Japan's east coast.

A spokesman at Tokyo Electric Power Co (TEPCO), operator of Fukushima, said some workers had been ordered to evacuate to higher ground after the quake, but that there was no damage or change in readings at radiation monitoring posts around the plant.

Ships were seen leaving a port south of the Fukushima Daiichi nuclear power station, where three reactors had meltdowns in 2011 after an earthquake and tsunami knocked out cooling and power, as a precaution after the tremor.

Buildings shook as far away as Tokyo, 230 km south of the nuclear plant.

The Japan Meteorological Agency issued a "yellow" warning shortly after the 1710 GMT quake that a small tsunami would reach the northeastern Japanese coast.

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 Fukushima plant in 2011 and devastated large swathes of Japan's eastern seaboard.