Japanese officials say radiation levels around the stricken Fukushima Dai-ichi power plant have reached levels that could damage health.

The stark announcement represents a dramatic deepening of the crisis and followed news of yet another explosion at the crippled plant and a fire at a fourth reactor within the same complex that resulted in the release of more radiation.

The Dai-ichi plant is the same facility where two previous explosions first stoked concerns of a possible meltdown and a major release of radiation.

Japanese Prime Minister Naoto Kan said in a nationally televised address that the level of radiation at the plant had "considerably risen."

"The level seems very high and there is still a very high risk of more radiation coming out," Kan said.

He warned people living within 30 kilometres of the Dai-ichi plant to stay indoors to avoid potential health risks from the radiation.

A fourth reactor at the Dai-ichi complex also caught fire, releasing more radiation. The fire has now been extinguished, according to Japan's nuclear safety agency.

"Now we are talking about levels that can damage human health," said Chief Cabinet Secretary Yukio Edano. "These are readings taken near the area where we believe the releases are happening. Far away, the levels should be lower."

Japan's nuclear safety agency said late Monday it suspects the blast at Unit 2 of the plant may have damaged the bottom of the container that surrounds the generator's nuclear core.

"A leak of nuclear material is feared," agency spokesman Shinki Kinjo said.

The plant's owner, Tokyo Electric Power Co., said the blast took place near the suppression pool in the reactor's containment vessel.

Plant employees evacuated

The company said it had evacuated non-essential power plant employees and acknowledged that radiation levels around the plant spiked after the blast.

It appears that this latest blast at the Unit 2 reactor was due to a buildup of hydrogen, like the previous two explosions.

Technicians had been desperately working to prevent uranium rods in the Unit 2 reactor from overheating after they were exposed twice on Monday.

The fuel rods were exposed after a steam vent in the reactor wouldn't open. Water levels were restored after they first dropped at the reactor, but the rods remained at least partially exposed Monday night after the second episode, officials said.

"Units 1 and 3 are at least somewhat stabilized for the time being," said Nuclear and Industrial Agency official Ryohei Shiomi. "Unit 2 now requires all our effort and attention," he said before the latest blast.

Prime Minister Kan told reporters earlier on Monday that his government — along with the Tokyo Power Electric Co. — will set up a joint response centre to better manage the nuclear crisis. Kan said he will personally lead operations at the centre.

Officials had declared states of emergency at six Fukushima reactors — three at Dai-ichi and three at the nearby Fukushima Daini complex — after Japan was hit Friday by a 9.0 magnitude earthquake (upgraded from 8.9) centred offshore and resultant tsunami. The main cooling systems and backup generators at the reactors were knocked out by the twin disasters.

Most of the attention in the past three days has been focused on Daiichi units 1 and 3. A complete meltdown — the melting of the radioactive core — could release radioactive contaminants into the environment and pose major, widespread health risks.

Japanese officials are using all available resources to monitor the situation, the country's ambassador to Canada said Monday.

"We are in close contact with not only American experts but also and especially with [the International Atomic Energy Association]... so that everybody knows what's going on," Kaoru Ishikawa told CBC News Network. "We are using the wisdom of all available engineers."

In another development, officials warned Monday that the death toll from the earthquake and resulting tsunami is likely to exceed 10,000.

Unit 3 reactor rocked by earlier explosion

Earlier Monday, the second explosion in three days rocked the Fukushima Dai-ichi plant, sending a massive cloud of smoke into the air and injuring 11 workers.

The blast at the Unit 3 reactor, which authorities had been trying to cool with seawater, triggered an order for hundreds of people to stay indoors, said Chief Cabinet Secretary Yukio Edano.

"Clearly the situation still remains serious as it was at Unit 1 on Saturday and Unit 3 on Sunday," McMaster University nuclear safety expert John Luxat told CBC News.

"The hydrogen explosion is an indication at these other two reactors that some fuel was exposed to become uncovered by water, heated up, and as a result of that [produced] hydrogen gas. The issue with the third reactor will be how long the fuel remains exposed, how hot it gets, and how much hydrogen is generated."

Operators knew the seawater flooding would cause a pressure buildup in the reactor containment vessel — and potentially lead to an explosion — but felt they had no choice if they wanted to avoid a complete meltdown. In the end, the hydrogen in the released steam mixed with oxygen in the atmosphere and set off the blast.

The inner containment shell surrounding the Unit 3 reactor was intact, Edano said, allaying some fears of the risk to the environment and public. But the outer building around the reactor appeared to have been devastated, with only a skeletal frame remaining.

More than 180,000 people have evacuated the area in recent days, and up to 160 may have been exposed to radiation — pouring misery onto those already devastated by the twin disasters.

U.S. vessel moved amid radiation fears

Earlier Monday, 17 U.S. military personnel involved in helicopter relief missions were found to have been exposed to low levels of radiation upon returning to the USS Ronald Reagan, an aircraft carrier about 160 kilometres offshore.

U.S. officials said the exposure level was roughly equal to one month's normal exposure to natural background radiation in the environment, and after scrubbing with soap and water, the 17 were declared contamination-free.

But as a precaution, the U.S. said the carrier and other U.S. 7th Fleet ships involved in relief efforts had shifted to another area.

While Japan has aggressively prepared for years for major earthquakes, reinforcing buildings and running drills, the impact of the tsunami — which came so quickly that not many people managed to flee to higher ground — was immense.

By Monday, officials were clearly overwhelmed by the scale of the crisis, with millions of people having spent three nights without electricity, water, food or heat in near-freezing temperatures.