Tokyo: Japanese Government officials have said meltdown may have occurred in two reactors at the Fukushima power station.
Japan's chief cabinet secretary Yukio Edano says it is possible that the radioactive cores of the two damaged Fukushima reactors have already started to melt.
He said: "We believe that there is a possibility that meltdown has occurred but it is inside the reactor so we can't see."
"However, we are acting, assuming that a meltdown has occurred and with reactor number 3, we are also assuming the possibility of a meltdown as we carry out measures."
Officials from Tokyo Electric Power (TEPCO), the plant's operator, say that pressure is rising inside reactor No. 3, after its emergency cooling system failed to operate.
A similar problem had led to a blast at the plant's reactor No. 1 on Saturday.
The officials say that after the blast, the plant lost the emergency cooling system at a second reactor and it started releasing air from the reactor container vessel to lower pressure inside it and avoid any serious damage.
TEPCO said that radiation levels around the nuclear plant have risen above the safety limit but it does not mean an "immediate threat" to human health.
About 170000 people have been evacuated from the area around the plant, to prevent exposure to harmful radiation.
Officials said at least 19 people who had been sent to a nearby hospital have been found exposed to radioactivity.
They also confirmed that four people working in the plant had been injured in Saturday's explosion, but their injuries were not life threatening.
TEPCO has been pumping seawater into the reactor in an effort to cool the core.
Friday's earthquake triggered the automatic shutdown of Fukushima's reactors. But the quake caused a power cut, which affected the water pumps that cool the reactor's core.