Residents speak to police officers as they leave for Yamagata Prefecture from Minamisoma, Fukushima prefecture, which is located within the 20 km (12 miles) evacuation zone around the crippled Fukushima Daiichi nuclear plant, in this photo taken by Kyodo April 21, 2011. [Photo/Agencies]

TOKYO -- Japan declared a 12-mile (20-kilometer) area evacuated around its radiation-spewing nuclear power plant a no-go zone on Thursday, urging residents to abide by the order for their own safety or possibly face fines or detention.

Chief Cabinet Secretary Yukio Edano said the order would take effect at midnight and was meant to prevent unrestricted entry into the mostly deserted area, which was ordered evacuated after last month's tsunami and earthquake wrecked the Fukushima Dai-ichi plant's power and cooling systems.

Under a special nuclear emergency law, people who enter the zone will now be subject to fines of up to 100,000 yen ($1,200) and possible detention of up to 30 days. Up to now, defiance of the evacuation order was not punishable by law.

"We beg the understanding of residents. We really want residents not to enter the areas," Edano said. "Unfortunately, there are still some people in the areas."

The new ruling was not due to any particular change in conditions inside the plant, which appear to have somewhat stabilized. Even under the best-case scenario, however, the plant's operator says it will take at least six months to bring its reactors safely into a cold shutdown.

Related readings:  Japan's tourism in tough situation amid nuclear crisis  6.0-magnitude quake hits off Japan east coast  ROK, China, Japan to meet on quake, trade deal

Almost all the zone's nearly 80,000 residents left when the area was evacuated on March 12, but police had not been able to legally block them from going back. Police contacted Thursday said they had no estimate of the exact number of people who have returned to the zone or who still might be living there.

Officials said the order was meant to limit exposure to radiation leaking from the plant, and to control entry to prevent theft.

Edano said authorities would arrange brief visits for residents, allowing one person per household to return by bus for a maximum of two hours to collect necessary belongings. Residents would be required to go through radiation screening, he said.

But visits will not be allowed in a two mile (three kilometer) area closest to the plant, said Hidehiko Nishiyama of Japan's Nuclear and Industrial Safety Agency, confirming reports that zone would be completely off-limits.

Details were still being worked out.

"We realize this is extremely inconvenient for residents, but we urge you to be patient," Edano told reporters in Tokyo.

Prime Minister Naoto Kan was visiting the region Thursday to meet with local officials and evacuees to discuss the plans for strict enforcement of the evacuation zone.

Kan also visited a nuclear crisis management center in Fukushima, giving a pep talk to workers there. He has been under fire from the opposition for the government's response to the nuclear crisis.

Fukushima's governor, who has also expressed frustration with the crisis, said he strongly urged Kan to ensure the government properly handles the disaster and related compensation issues.

"I told the prime minister that I strongly hope that evacuees can return home as early as possible," said the governor, Yuhei Sato.