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 Japan's Disaster Countermeasures Basic Law, people who enter the zone would be subject to fines of up to 100,000 yen ($1,200) and possible arrest. 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."
Almost all the zone's nearly 80,000 residents left when the area was evacuated on March 12, but police have 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.
Japan Earthquake Triggers Tsunami
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.
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, who will also visit a nuclear crisis management center during his Thursday trip, has been under fire from the opposition for the government's response to the nuclear crisis.
Edano suggested Wednesday that plant operator Tokyo Electric Power Co. should have been better prepared.
"Aside from the question of whether the accident could have been predicted, there was not sufficient preparation based on an anticipation, and there is no mistake about that," he said. "We urge all nuclear operators to immediately take any possible precaution based on the lesson from the Fukushima nuclear accident, and not wait until details of the accident are examined."
Associated Press writers Eric Talmadge in Fukushima and Malcolm Foster and Shino Yuasa in Tokyo contributed to this report.