The Nuclear and Industrial Safety Agency told all power companies that run nuclear plants to ensure they had at least two emergency diesel generators for each reactor, whether or not it is operating.

Under safety rules in force up till now, Japanese nuclear plant operators have been required to have just one back-up generator for each inactive reactor. A requirement for two generators for active reactors is unchanged.

Hidehiko Nishiyama, the agency's deputy director general, said power outages -- even if of short duration -- were a serious issue, as highlighted in a risky situation at Higashidori nuclear plant after a quake on Thursday.

"We have said nuclear plants are definitely safe... and I myself believed in it," he told a news conference.

"But in the light of this experience, we need to review safety standards from all angles."

When a 7.1-magnitude aftershock rattled Japan late Thursday, the Higashidori nuclear plant, operated by Tohoku-Electric Power Co., had to rely on just one diesel generator, as two others were undergoing maintenance.

Several hours later, workers found a large amount of light oil leaking from the generator and had to stop it. They were only able to keep electricity on because by that time an external power line had been recovered.

As a result, key cooling systems at the plant had to rely on a single power source for about an hour. If this power source had failed, a situation like the one at the crisis-hit Fukushima Daiichi plant was a possibility.

The magnitude 9.0 earthquake on March 11 sent a 14-metre tsunami slamming into the Fukushima nuclear plant, knocking out cooling systems for four of six reactors and triggering the world's worst nuclear crisis in 25 years.

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