Radiation measurements at the Fukushima Daiichi nuclear plant were 1,000 times higher than normal after the massive earthquake in northern Japan, the Kyodo news agency reported early Saturday, citing Japan's nuclear safety agency.
There were concerns that radioactive steam may have escaped the plant due to high pressure inside an overheating reactor. The earthquake damaged power and water supplies and disrupted the reactor's cooling systems, dpa reported.
Japanese Prime Minister Prime Minister Kan Naoto on Saturday extended evacuations to a 10-kilometre radius around the plant. Immediately after the earthquake, authorities had set a 2-kilometre evacuation zone while telling residents within 10 kilometres to stay inside their homes.
Naoto toured the disaster area by helicopter.
Aftershocks on Saturday were continuing to rattle traumatized survivors.
More than 1,000 people were feared dead after Friday's 8.9- magnitude earthquake hit north-eastern Japan, generating a 10-metre- high tsunami that swept away people, cars, boats, crops and buildings.
It was the strongest tremor ever recorded in quake-prone Japan.
The quake generated tsunami alerts across the Pacific, as far away as Chile.
Concerns had been raised earlier Friday after the electrical system that supplies the cooling system failed at the Fukushima Daiichi nuclear plant, operated by the Tokyo Electric Power Co. The reactor's radioactive core needs continued cooling to prevent a meltdown.
Mobile power supplies were delivered to the plant to restore the system, which had been operating on batteries, the industry news outlet World Nuclear News said, citing an official statement.
US Secretary of State Hillary Rodham Clinton said the US Air Force in Japan had transported coolant to the plant.