Japan issues tsunami alert after strong quake (VIDEO)

A 6.8-magnitude earthquake jolted northeastern Japan off Fukushima prefecture and prompted a tsunami warning on Friday, the Japan Meteorological Agency said.

High-speed bullet trains were halted and highways closed after the quake, public broadcaster NHK said.

But the tsunami advisory for the coastal areas of Fukushima and Miyagi prefectures was lifted after no waves were sighted, Reuters reports.

The epicenter of the quake, which hit at 2:36 p.m., was off the coast of Fukushima prefecture, slightly south of where the massive 9.0 magnitude earthquake struck on March 11, according to the Japan Meteorological Agency.

The quake — the strongest on record in Japan — and a massive tsunami left about 20,400 dead or missing and triggered the world's worst nuclear crisis in 25 years at the Fukushima Dai-ichi nuclear power plant.

The Tokyo Electric Power Co. said there were no abnormalities in key equipment at the Fukushima Dai-ichi plant, the Washington Post reports, quoting Chie Hosoda, an official with the plant operator. Some of the plant’s workers assigned to the coastal side of the facility temporarily retreated inside the building, she reportedly said.

Tohoku Electric Power, meantime, said there were no abnormalities at its Onagawa nuclear power plant, which has been shut since the March disaster, Reuters reports.

Although the authorities said Friday that any tsunami waves would likely be limited to 20 inches (50cm) — compared with the more than 30-meter tsunami that struck some areas after the March 11 quake — announcers on television urged residents in coastal areas to head for higher ground.