The ban, which gives legal weight to an existing exclusion zone, comes after police found more than 60 families living in the area and residents returning to their abandoned homes to collect belongings.

Prime Minister Naoto Kan announced the no-entry area, due to be enforced from midnight (1500 GMT), on a visit to Fukushima prefecture, where thousands now live in evacuation shelters, almost six weeks after the March 11 quake.

The nuclear plant, where reactor cooling systems were knocked out, has been hit by a series of explosions and leaked radiation into the air, ground and sea in the world's worst nuclear disaster since Chernobyl 25 years ago.

More than 85,000 people have moved to shelters from areas around the plant, including from a wider 30-kilometre zone, where people were first told to stay indoors and later urged to leave.

"The plant has not been stable," Kan's right-hand man, top government spokesman Yukio Edano, said at a Tokyo news conference. "We have been asking residents not to enter the area as there is a huge risk to their safety."

The ban can be enforced with detentions or 100,000 yen ($1,200) fines.

One member of each household within the 20-kilometre no-entry area will be allowed to make a two-hour return visit to their home to pick up personal belongings, wearing protective suits and dosimeters.

The trips on buses will start within days and run for one or two months, but exclude areas within three kilometres of the crippled Fukushima Daiichi plant, where the radiation risk is deemed too high, Edano said.

"They are advised to keep the belongings they take out to a minimum," said Edano, the chief cabinet secretary, adding that all visitors to the no-go area would be screened afterwards for radiation exposure.

There are 27,000 households within the 20-kilometre zone, said broadcaster NHK.

They are likely to have to stay away for some time, after plant operator Tokyo Electric Power Company (TEPCO) said Sunday that it did not expect a "cold shutdown" of all six reactors for another six to nine months.

Kan asked for residents' understanding when he flew to the prefecture by military helicopter and met Fukushima governor Yuhei Sato, before visiting local evacuation shelters.

"We have been doing our best to have people return to their hometowns and resume their lives there," Kan told local officials during his trip. "And we will make further efforts to realise this."

Sato later told reporters he had demanded "that Tokyo Electric Power as well as the government cover compensation responsibly, including damage caused by harmful rumours, so that evacuees can return as quickly as possible."

Many of those living in crowded shelters voiced frustration.

"We are all worried because we don't know how long this will go on for," a woman told NHK. "I want the government to tell us when this will end."

A man from Narahamachi, a town of around 8,000 in Fukushima, said: "The roof of my house is probably gone, but I can't even fix it. It's just unacceptable that we will only be allowed to go back for an hour, two hours."

A schoolboy at one shelter said: "I want to go back, even if it's a one-off thing for now. I need my study materials. I play baseball, so I want to bring back my baseball gear."

TEPCO, meanwhile, said that some 520 tonnes of water that leaked into the sea from a crack in a storage pit at the power station had reached about 4,700 terabecquerels -- 20,000 times above the plant's legal annual limit.

Environmental group Greenpeace said it was sending its Rainbow Warrior flagship from Taiwan to Japan to test seawater and marine life near the plant, starting as early as next Wednesday.

Aftershocks continue to rattle Japan after the massive earthquake and tsunami that hit the northeast of the country on March 11, leaving more than 27,000 people dead or missing.

A strong earthquake of magnitude 6.0 jolted buildings in Tokyo late Thursday but appeared to cause no damage and there was no tsunami warning.

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