A strong earthquake registering a preliminary magnitude of 6.8 has struck off Japan's northeastern coast, triggering a tsunami alert.

Japan's Meteorological Agency said the earthquake hit at 2.36pm local time and was centred slightly south of where a massive magnitude 9.0 quake struck on March 11.

The agency issued a tsunami advisory, predicting waves of 1.5ft along the coast of Miyagi and Fukushima prefectures, where nuclear reactors crippled in the March quake are located.

In Tokyo, buildings swayed slightly.

Announcers on television urged residents in coastal areas to head for higher ground, but about half-an-hour after the quake, there were no reports of a tsunami.

In Onagawa, about 210 miles north of Tokyo, town official Hironori Suzuki said there were no immediate reports of damage or injuries. There was no visible swelling of the ocean.

"It was a rather big one, perhaps it was because we are still in a makeshift office," Suzuki told public broadcaster NHK.