Major aftershock rocks Japan and tsunami warning issued, but later lifted

A major aftershock rocked northeast Japan late on Thursday and a tsunami warning was issued for the coast devastated by last month's massive quake and tsunami that crippled a nuclear power plant.

The tsunami warning was later lifted and no damage from the quake, measured at magnitude 7.4 by the Japan Meteorological Agency, was detected at the Fukushima Daiichi plant, operator Tokyo Electric Power Co (TEPCO) said.

Workers struggling to bring the plant under control were evacuated soon after the aftershock struck.

Large parts of northern Japan, where infrastructure was severely damaged by the March 11 quake and tsunami, were without electricity following the latest of many aftershocks, the biggest since last month's killer quake.

In the capital, Tokyo, buildings shook. "It started out as nothing much, then the building started swaying quite strongly," a Reuters witness said.

NHK public television cited police as saying seven people had been injured, two of them seriously.

Last month's disaster has disrupted industry and affected supply chains around the world but it was not immediately clear if Thursday's aftershock would compound those problems.

At the Fukushima nuclear plant, TEPCO said it was continuing to inject nitrogen into reactor No.1 after no irregularities were reported.

Engineers, who sealed a leak this week that had allowed highly radioactive water into the sea, are pumping nitrogen into one reactor to prevent the risk of a hydrogen gas explosion, and want to start the process in another two reactors.

There were no abnormalities in radiation levels around Tohoku Electric's Onagawa nuclear power plant, where fuel rods are being cooled with just one outside power source, Japan's nuclear safety agency said.

As well as Fukushima Daiichi and Onagawa, nuclear power plants Higashidori in Aomori prefecture, Tokai No.2 in Ibaraki prefecture, and Fukushima Daini have been out of operation since the March quake.

No abnormalities were reported at those plants after Thursday's quake, which the meteorological agency said was an aftershock from last month's magnitude 9.0 quake which triggered huge tsunami waves.

About 28,000 people were killed or are missing.

Japan's neighbors have sounded increasingly alarmed over the risk of radiation from the plant, while tourists are staying away in what should be the peak season, and the country seeks ways to cut power use.