Japan added to the evacuation zone near a stricken nuclear plant, as a powerful aftershock rattled the nation a month after its biggest recorded earthquake wrought devastation.

The move to restrict pockets beyond the current 20-kilometre evacuation area, came amid assurances Monday from chief government spokesman Yukio Edano that the chances of the atomic crisis deepening were "significantly smaller" than one or two weeks after the March 11 earthquake and tsunami.

However, the Kyodo news agency reported early Tuesday that Japan was considering raising the severity of the accident to seven -- the top level on an international scale -- from five.

Kyodo, citing unnamed government sources, said radiation levels measured by the country's Nuclear Safety Commission prompted the deliberation of an upgrade to the highest level -- previously given only to the 1986 Chernobyl disaster.

In a reminder of ongoing uncertainty across Japan, workers at the crippled Fukushima Daiichi plant were forced to run for safety after the latest of hundreds of powerful tremors since the 9.0 magnitude quake struck on Monday.

Japan's meteorological agency warned that a tsunami wave up to one metre (three feet) high could hit the coast near the power station after the 6.6-magnitude shock, before cancelling the alert less than an hour later.

One man died in Inaraki Prefecture in Monday's aftershock, Jiji press said, as thousands marked a month since the massive quake created a debris-laden wave that crushed towns, killed at least 13,000 and left about 14,000 missing.