The Japanese government has declared a state of atomic power emergency after a powerful 8.9-magnitude earthquake struck off Japan's northeastern coast Friday, spawning a tsunami as high as 10 metres.

The governement said no radiation leaks have been detected at or near any nuclear power plants as of Friday evening.

Four nuclear power plants closest to Friday's powerful 8.9-magnitude earthquake that struck off Japan's northeastern coast have been safely shut down, the International Atomic Energy Agency (IAEA) said.

According to the Japan's industry ministry, a total of 11 nuclear reactors were automatically shut down at the Onagawa plant, Fukushima No. 1 and No. 2 plants and Tokai No. 2 plant after the massive earthquake.

Prime Minister Naoto Kan made the declaration so authorities can easily establish an emergency task force to deal with the situation, Chief Cabinet Secretary Yukio Edano told a news conference.

There are no reports of leakage from any nuclear power plants at the moment and no signs of any leakage, Edano said. We have declared a nuclear emergency state to take every possible precaution, he said. Let me repeat that there is no radiation leak, nor will there be a leak. We ask residents in the areas near power plants to act calmly.

The quake measuring 8.9 on Richter scale struck about 382 kilometres (237 miles) northeast of Tokyo, the US Geological Survey said.

Around 20 countries and numerous Pacific islands have been issued tsunami warning after the powerful earthquake, the National Weather Service said.