A minor tsunami hit Miyagi prefecture in Japan early Saturday after a strong 6.8-magnitude quake jolted the country's northeastern Pacific coast, prompting advisories for regions including around the crippled Fukushima nuclear plant.

A tsunami of 20 centimetres was observed at 5.12 am (2012 GMT Friday) in Ishinomaki, Miyagi prefecture, the Japan Meteorological Agency said.

The agency had issued a tsunami advisory for Miyagi as well as neighbouring Fukushima and Iwate prefectures, warning that a wave of up to one metre (3.3 feet) could impact their Pacific coastlines after the quake.

Large areas of the coastline covered by the advisory are still recovering from the 2011 quake and tsunami disaster that killed more than 18,000 people and triggered a meltdown at the Fukushima Daiichi nuclear plant.

Plant operators Tokyo Electric Power (TEPCO) said there were no immediate reports of abnormality after Saturday's quake.

"We have not seen any damage or any change in radiation gauges after the quake," said TEPCO spokesman Masahiro Asaoka.

"Today's operation has yet to start but we ordered workers to evacuate to high places," Asaoka said.

The meteorological agency advised people to leave the coast immediately, while Japan's public broadcaster NHK said some local authorities had issued evacuation advisories to their residents.

Kyodo said the city of Iwate had issued an evacuation advisory.

The United States Geological Survey said the quake struck around 129 kilometres (79 miles) east south-east of the city of Namie, an estimated 284 kilometres (176 miles) east north-east of the capital Tokyo, at 4.22 am local time (1922 GMT Friday).

The Fukushima plant's cooling systems were swamped by the 2011 tsunami, sparking reactor meltdowns and radiation leaks in the worst atomic crisis in a generation.

Tens of thousands of people were evacuated from around the plant, with decommissioning of the site expected to take decades.

The utility is struggling to handle a huge -- and growing -- volume of contaminated water.

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On Friday, the crippled plant was skirted by tropical storm Neoguri. Workers had scrambled to insulate the plant from any storm damage, but Neoguri had little impact on the site as it headed out into the Pacific.

Japan is situated at the conjuncture of several tectonic plates and experiences a number of relatively violent quakes every year.

But thanks to strict building codes, even powerful quakes that might wreak havoc in other countries frequently pass without causing much damage.