A state of emergency has been declared at a second Japanese nuclear power plant.
Excessive radiation levels have been recorded at a facility in Onagawa following Friday's earthquake and tsunami.
There has already been an explosion at a nuclear plant in Fukushima with a neighbouring reactor also at risk.
Japan's prime minister has said the country is facing its worst crisis since the end of the Second World War.
Japan’s nuclear crisis has intensified as authorities raced to combat the threat of multiple reactor meltdowns and evacuated more than 170,000 people after the devastating earthquake and tsunami.
Amid fears of possible radioactive contamination, nuclear plant operators are trying to keep temperatures down in a series of reactors – including one where officials feared a partial meltdown could be under way – to prevent the disaster from growing worse.
Chief Cabinet Secretary Yukio Edano also said that a hydrogen explosion could occur at Unit 3 of the Fukushima Dai-ichi nuclear complex, the latest reactor to face a possible meltdown.
This follows a blast the day before in the power plant’s Unit 1, as operators attempted to prevent a meltdown there by injecting sea water into it.
“At the risk of raising further public concern, we cannot rule out the possibility of an explosion,” Mr Edano said.
“If there is an explosion, however, there would be no significant impact on human health.”
More than 170,000 people had been evacuated as a precaution, though Mr Edano said the radioactivity released into the environment so far was so small it did not pose any health threats.
Mr Edano said neither Fukushima Dai-ichi reactor was near the point of complete meltdown, and he was confident of escaping the worst scenarios.
A complete meltdown – the collapse of a power plant’s systems and its ability to keep temperatures under control – could release uranium and dangerous contaminants into the environment and pose major, widespread health risks.
Up to 160 people, including 60 elderly patients and medical staff who had been waiting for evacuation in the nearby town of Futabe, and 100 others evacuating by bus, might have been exposed to radiation, said Ryo Miyake, a spokesman from Japan’s nuclear agency. The severity of their exposure, or if it had reached dangerous levels, was not clear. They were being taken to hospitals.
Mr Edano said operators were trying to cool and decrease the pressure in the Unit 3 reactor, just as they had the day before at Unit 1.
“We’re taking measures on Unit 3 based on a similar possibility (of a partial meltdown),” Mr Edano said.
Unit 3 at the Fukushima plant is one of the three reactors that had automatically shut down and lost cooling functions necessary to keep fuel rods working properly due to power outage from the quake. The facility’s Unit 1 is also in trouble, but Unit 2 has been less affected.
On Saturday, an explosion destroyed the walls of Unit 1 as operators desperately tried to prevent it from overheating and melting down.
Without power, and with its valves and pumps damaged by the tsunami, authorities resorted to drawing sea water mixed with boron in an attempt to cool the unit’s overheated uranium fuel rods. Boron disrupts nuclear chain reactions.
The move likely renders the 40-year-old reactor unusable, said a foreign ministry official briefing reporters. Officials said the sea water will remain inside the unit, possibly for several months.