High radiation levels at Japan's Fukushima 1 nuclear plant have raised fears that a reactor core may have been damaged in the earthquake and tsunami that is now confirmed to have killed 10,000 people.
Two workers at the site needed hospital treatment for radiation burns
"Radioactive substances have leaked to places far from the reactor," said Hideyuki Nishiyama, a spokesman for Japan's nuclear safety agency.
"As far as the data show, we believe there is a certain level of containment ability but it's highly possible that the reactor is damaged."
Concerns were raised after two workers at the plant were taken to hospital with radiation burns after stepping in highly radioactive water in the basement of the number three reactor's turbine building.
The number three reactor is of particular concern because it is the only one of six at the facility to use a potentially volatile mix of uranium and plutonium.
A hydrogen explosion badly damaged the unit's outer building on March 14, and a partial meltdown is also suspected.
The plant is emitting radiactive vapour and Japan's government has now urged people still living within 18 miles (30km) of the plant to leave the area after previously telling them to remain indoors.
Chief cabinet secretary Yukio Edano said: "I cannot rule out the possibility that the government will issue an evacuation order for this area if the radiation level goes up further."
Japanese prime minister Naoto Kan warned that the situation at Fukushima 1 was "grave and serious".
He also apologised to farmers and business owners around the plant for the damage caused to their livelihoods
Hundreds of thousands of residents were evacuated from within a 12-mile radius of the site in the days after the March 11 earthquake and tsunami disabled its crucial cooling systems.
Official figures now put the number of confirmed dead in the disaster at 10,000.
About 17,500 are still listed as missing and Japanese officials have said there is little hope of finding many more survivors.
The elderly, a large demographic in Japan, were especially hard hit.
Two-thirds of the victims were older than 60, according to an initial survey of 2,800 identified victims by the Yomiuri Shimbun newspaper.
About 660,000 households in the battered northeast region are without water and more than 200,000 do not have electricity.