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.

Much of Japan fell silent at 2:46 pm as people across the country remembered the victims of Japan's worst catastrophe since World War II.

In the broken city of Kesennuma, soldiers looking for corpses among the piles of rubble paused to pay their respects.

Cold rain fell and the stench of rotting matter hung in the air as the men bowed their heads briefly before resuming their gruesome search.

Tokyo said it was augmenting the evacuation area around Fukushima Daiichi because of long-term health worries, even as the government said the danger of a large leak of radioactive materials was fading.

"The possibility that the situation at the nuclear plant will deteriorate and lead to new leakage of massive radioactive materials is becoming significantly smaller," chief government spokesman Yukio Edano told reporters.

"We believe the risk of that has become significantly smaller compared to one or two weeks after the earthquake," Edano said, a month to the day after the twin disasters knocked out cooling systems at the Fukushima plant.

He later told reporters the government was concerned over the effect of long-term exposure to radiation and would be ordering people to leave certain areas more than 20 kilometres (12 miles) from the plant, which are currently outside the exclusion zone.

But Edano said a uniform extension of the zone was not appropriate.

"There are some places where cumulative levels of radiation are increasing depending on climate and geographical conditions, even outside of the 20-kilometre radius circle," he said.

Environmental pressure group Greenpeace welcomed the move, but said it did not go far enough.

Monday's aftershock hit 81 kilometres (50 miles) south of Fukushima city and 163 kilometres northeast of Tokyo at a depth of 10 kilometres, causing buildings in the capital to shake.

Tokyo Electric Power said that the tremor briefly knocked out the power supply to reactors at Fukushima Daiichi, before it was restored and the injection of cooling water was resumed.

Japan's Meteorological Agency warned that more magnitude 7 aftershocks could rattle the country, according to a report by Kyodo.

Masataka Shimizu, the president of Tokyo Electric Power Co. (TEPCO), on Monday visited Fukushima and apologised for the atomic emergency engulfing the area, which has affected farm produce and sparked international concern.

Shimizu had wanted to go to the offices of the Fukushima prefecture government in the hope of meeting governor Yuhei Sato, but a local official said no meeting had taken place.

The official gave no reason, but Sato has previously refused to meet the boss of the embattled utility.

The quake hit at 5:16 pm (0816 GMT), as Prime Minister Naoto Kan prepared to deliver a live address to the nation to mark one month since the giant earthquake. The address was postponed until Tuesday.

An open letter from Kan appeared in some of the world's leading papers Monday, thanking people around the globe for their support.

US Secretary of State Hillary Clinton will visit Japan in a show of support for the US ally as it recovers from a devastating earthquake, the State Department announced Monday (US time).

Clinton will travel to Tokyo on Sunday, after stops in South Korea and in Germany where she is attending a NATO conference, a State Department spokesman said.