(WATCH: Footage of the new explosion at the Fukushima plant.)
There's been another explosion at the Fukushima Daiichi nuclear power plant damaged by Japan's massive earthquake and tsunami.
Officials said six people were injured but the core container was not damaged following the explosion this morning at reactor number 3 at the plant.
Meanwhile, authorities said the cooling system of a third reactor at the same power station had failed.
The March 11 quake triggered the world's worst nuclear accident since Chornobyl in 1986 when a reactor at the Fukushima Daiichi plant 240 kilometers north of Tokyo exploded after its cooling system failed. Officials raced to prevent fuel rods from overheating by leaking radioactive gas into the air to relieve pressure in the unit.
Concerns over a nuclear power plant destabilized by the March 11 earthquake and tsunami in Japan are on the rise, with a top Japanese official saying that a partial meltdown of a reactor is highly possible.
The plant faced the threat of new radiation leaks on March 13 after the cooling system failed at a second reactor, where engineers were pumping in seawater to prevent a meltdown, which the authorities now say is unlikely.
Ambulances and buses helped evacuate thousands of people after the plant's operator Tokyo Electric Power (TEPCO) said radiation levels around the Fukushima plant had risen above the legal safety limit. However they said the radiation did not present an "immediate threat" to human health.
TEPCO said it was preparing to vent steam to relieve pressure in the plant's number three reactor, where a meltdown would probably be more serious than in other reactors because it uses both uranium and plutonium for fuel. Speaking at a news conference in Tokyo, Japanese Cabinet Secretary Yukio Edano warned of a rise in radiation during the procedure.
"If there is leak from reactor number three, that is part of the venting process, which is a controlled release," Edano said.
The full extent of the damage may not be known until workers open the reactors, which could take months.
In the city of Koriyama, outside the 20 kilometer safety zone set up around the nuclear plant, city officials in protective clothing used Geiger counters to check evacuees for radiation. Japan's nuclear safety agency said 160 people could have been exposed to radiation.
The earthquake was one of the biggest in recorded history. Many buildings appeared to withstand the quake, but were brought down by the tsunami that created scenes of utter devastation. Buildings, cars, trains, and boats made up part of a jumble of flood wreckage strewn across large areas.
Japanese Prime Minister Naoto Kan has called for the country to come together to overcome what he called the worst crisis facing the nation since World War II.
"In terms of this earthquake and tsunami, I am very confident the Japanese people will come together and overcome this difficulty," Kan said.
A massive relief operation is under way in northern Japan amid fears that more than 10,000 people may have died in just one of the areas hit by the quake and devastating tsunami it triggered.
Rescue workers have found scenes of utter devastation in the northern Miyagi prefecture, which bore the brunt of the 8.9-magnitude quake and the tsunami that followed, demolishing towns and devastating communications, transportation and other infrastructure.
WATCH: Video footage posted on YouTube shows the tsunami hitting the fishing port of Miyako in the Iwate prefecture.
The authorities say 10,000 are believed to be dead or missing in the Miyagi prefecture alone. Television pictures showed many of those who survived the quake and tsunami wrapped in blankets, some sobbing.
In the province's port of Sendai, where newly released video showed a 10-meter high tidal wave surging through the town on March 11, resident Nobuo Ishigamori stood in front of burning scrap.
"What happened was beyond anyone's expectations. No one thought the damage would be this bad. There have been a lot of big earthquakes, but this is the first of this size and it makes you realize that you never know when disaster can strike," Ishigamori said.
People in the region are in shock, but reports say they're not panicking as strong aftershocks continue to hit Japan's main island. Residents have largely been organized in their attempts to get to safety and find food as rescue workers mount a large-scale effort to find survivors.
The government says 3,000 people have been rescued so far. More than 50 countries have offered help, including the United States, which has three warships off Japan's coast.
The Kyodo news agency reports about 300,000 people have been evacuated nationwide. It said 5.5 million people were without power, and that 20,800 buildings had been destroyed or damaged.
Some 100,000 troops from Japan's Self-Defense Forces have been ordered to help with the rescue effort in Japan's largest military mobilization since World War II.
Officials are still getting a sense of the scale of the damage. But newspapers and anti-nuclear NGOs have already blasted the government for being poorly prepared for the crisis, which threatens to derail Japan's fragile economic recovery.
Written by Gregory Feifer with agency reports