Japan’s nuclear authorities warned on Saturday that quake-hit atomic plant Fukushima No 1, about 250 kilometres Northeast of Tokyo, "may be experiencing a nuclear meltdown", media said.
Kyodo News agency said radioactive caesium had been detected near Fukushima plant, citing the Japanese nuclear safety commission.
The cooling system of the plant was damaged in the massive earthquake that struck the region 24 hours earlier, leaving the government scrambling to fix the problem and evacuate more than 45,000 residents within a 10-kilometre radius.
Thousands were also evacuated from near a second plant, Fukushima No 2, which also suffered damage to its cooling system.
Parts of the No 1 reactor's nuclear fuel rods were briefly exposed to the air on Saturday after cooling water levels dropped through evaporation, and a fire engine was pumping water into the reactor, Jiji Press reported.
The water levels were recovering, said operator Tokyo Electric Power (TEPCO), according to Jiji.
A TEPCO spokesman told AFP that "we believe the reactor is not melting down or cracking. We are trying to raise the water level."
Public broadcaster NHK quoted Japan's nuclear and industrial safety agency as saying that metal tubes which contain uranium fuel may have melted.
The broadcaster quoted Tokyo University professor Naoto Sekimura as saying that "only a fraction of the fuel may have melted but the reactor had already been stopped and is being cooled. I urge residents to act calmly."
The government on Friday declared an atomic emergency amid growing international concern over its reactors after the 8.9 magnitude earthquake, the biggest in Japan's history, unleashed tsunamis that swept all before them.
When Friday's massive quake hit, the plants immediately shut down, along with others in quake-hit parts of Japan, as they are designed to do — but the cooling systems failed, the government said.
When reactors shut down, cooling systems must kick in to bring down the very high temperatures. These systems are powered by either the external electricity grid, backup generators or batteries.
The fear is that fuel rods, if they can't be cooled, become exposed and release radioactivity, leading to a meltdown.
A meltdown is a severe nuclear power plant failure in which the reactor core is unable to properly cool, leading nuclear fuel assemblies to overheat and melt, potentially releasing radioactive materials into the environment.
Tokyo electric power on Saturday vented radioactive vapour at five reactors between both plants to release building pressure.
On Friday the US Air Force, which has many bases in Japan, delivered coolant to a Japanese nuclear plant, US secretary of state Hillary Clinton said, without specifying which plant.