A radiation leak is feared after Japan's Nuclear Safety Agency reported a third explosion at Unit 2 of the stricken Fukushima Daiichi nuclear-power plant in the country's northeast.
Shinji Kinjo, an agency spokesman, said that "a leak of nuclear material is feared", after the explosion was heard at 6:10am local time (21:10 GMT) on Tuesday.
A fire which broke out following the explosion has apparently been extinguished, Kyodo news agency and other media quoted the power station operator as saying.
The troubles at the Fukushima Daiichi nuclear plant began when a massive earthquake and tsunami in Japan's northeast on Friday knocked out power, crippling cooling systems needed to keep nuclear fuel from melting down.
Radiation levels are falling at the nuclear plant, the Japanese government said, after an earlier spike in radiation. Yukio Edano, the chief cabinet secretary, told reporters that radiation levels had fallen dramatically to 596.4 microsieverts per hour as of 06:30 GMT.
That level is almost 700 times less than the levels reported in the morning, after two fresh blasts at the complex.
Nonetheless, Naoto Kan, the Japan prime minister, warned people within 30km of the Fukushima plant to stay indoors. Low-level radioactive wind from the reactor could reach Tokyo, the Japanese capital, within 10 hours, based on current winds, the French embassy said in a statement on its Japanese-language website on Tuesday.
Al Jazeera’s Steve Chao, reporting from Yamagata, said a no-fly zone has been established in a 30 km radius over the Fukushima nuclear plant.
Tokyo Electric Power Co (TEPCO), the plant's operator, said the explosion occurred near the suppression pool in the reactor's containment vessel. The pool was later found to have a defect. TEPCO said some employees of the power plant were temporarily evacuated following Tuesday's explosion.
An agency spokesman, Shigekazu Omukai, said the nuclear core of Unit 2 was not damaged in the explosion. But the agency said it suspects the bottom of the container that surrounds the generator's nuclear core might have been damaged.
Al Jazeera's Harry Fawcett, reporting from Ichinoseki, in northeast Japan, said: "People didn't know what was happening and they wonder what they can do. Some say that they can't get out due to lack of fuel.
"We know that there was a sound of explosion at Unit 2, where there are significant numbers of fuel rods submerged in water.
"The government is sticking to the line that radiation is within safety levels, but it is a fast-changing situation."
On Monday, a second explosion had rocked the same unit of the Fukushima Daiichi power plant, sending a plume of smoke into the air.
But the International Atomic Energy Agency (IAEA) said that the reactor had not been damaged. The World Health Organisation said there was a minimal public heath risk.
TEPCO said afterwards that fuel rods at one of the reactors had become fully exposed again, meaning the water being pumped in to cool the reactors is evaporating due to the heat.
Japanese nuclear officials worked to quell concerns and announced the distribution of 230,000 units of stable iodine. Iodine can be used to help protect against thyroid cancer in the case of radioactive exposure.
Yukio Edano, Japan's chief cabinet secretary, said that a large-scale radiation leak was unlikely. He said the reactor's inner containment vessel holding the nuclear fuel rods was intact, allaying some fears of the risk to the environment.
The accidents - injuring 15 workers and military personnel and exposing up to 190 people to elevated radiation - have compounded the challenges faced by the Japanese government as it struggles to help survivors of the quake-tsunami disaster that flattened entire communities.
Police revised on Tuesday the official death toll, putting it at 2,414 dead, with thousands more missing.
Spectre of Chernobyl
Koichiro Genba, Japan's national strategy minister, said there was "absolutely no possibility of a Chernobyl" - a reference to the 1986 explosion at a Soviet reactor which spread radiation over swathes of Ukraine, Belarus, Russia and northern Europe and is estimated by UN agencies to have caused the deaths of thousands of people.
But some people in the affected area said they were worried at the prospects of nuclear radiation. Twenty people have tested positive for radiation exposure and that number looks likely to rise.
TEPCO said in a press release that the blast was believed to be a hydrogen explosion at the plant's No 3 reactor and that 11 workers were injured. The first explosion happened at the same plant on Saturday, at the reactor No 1.
It also said the impact of radioactive materials to the outside environment was under investigation.
At the Fukushima plant, efforts have continued to cool the reactors with a mixture of seawater and boric acid - an untested method - as a last resort.
Against this backdrop of continued safety concerns, foreign aid has begun to arrive for the tsunami-affected region of Japan. Up to 70 countries have offered assistance, with help coming not only from allies like the US but also countries with more strained relations like China.
Millions of people spent a third night without water, food or heating in near-freezing temperatures along the devastated northeastern coast.
Search intensifies for Japan survivors [Al Jazeera]
In many areas there is no running water, no power and four- to five-hour waits for petrol. People are suppressing hunger with instant noodles or rice balls while dealing with the loss of loved ones and homes.
“People are surviving on little food and water. Things are simply not coming,'' Hajime Sato, a government official in Iwate prefecture, said.
In another grim development, hundreds of bodies washed ashore on Monday along the northeastern coastline, the area worst hit by the tsunami.
A Japanese police official said 1,000 bodies were found scattered across the coastline of Miyagi prefecture.
Kyodo, the Japanese news agency, reported that 2,000 bodies washed up on two shorelines in Miyagi.
The US Geological Survey upgraded on Monday the earthquake to magnitude 9.0, from 8.9, making it the world's fourth most powerful since 1900.
Hiromichi Shirakawa, chief economist for Japan at Credit Suisse, has estimated the economic loss will probably be around $171-183bn just to the region hit by the twin disasters.