A strong earthquake has struck near the troubled nuclear power plant at the centre of Japan's radiation crisis, the meteorological agency said.
The agency said there was no fear of a tsunami following the quake, whose magnitude was recorded at 6.0 by the agency today.
Public broadcaster NHK said there was no immediate report of further damage to nuclear power plants in Fukushima, including the Fukushima No. 1, which was crippled by the March 11 quake and tsunami.
The number of people confirmed dead or missing after March 11 has topped 21,000.
Engineers have partially restored power to one of the control rooms at the plant, as radioactivity in the sea fuelled anxiety over food safety.
An external electricity supply has now been linked up to all six reactors at the Fukushima No 1 power station, more than a week after a 14-metre tsunami crippled the ageing facility.
In another small step towards regaining control of the plant, the lights came back on in the control centre of the number three reactor, making it easier for workers toiling to get the vital cooling systems working again.
"As of 10.43pm [local time] the control centre for reactor number three had its lighting on," an official from plant operator Tokyo Electric Power Co [Tepco} told reporters late yesterday.
The number three reactor is a particular concern because it contains a potentially volatile mixture of uranium and plutonium.
The twin quake and tsunami disaster, Japan's worst crisis since World War II, has left nearly 23,000 people dead or missing, with entire communities along the northeast coast swept away.
Now the shell-shocked nation faces an invisible threat from radiation seeping from the Fukushima No 1 plant, which lies just 250km from the greater Tokyo area and its 30 million inhabitants.
The health ministry advised residents in five towns or cities in Fukushima prefecture not to use tap water to make formula milk and other drinks for babies due to abnormally high radiation levels.
The government also ordered increased inspections of seafood after radioactive elements were detected in the Pacific Ocean near the Fukushima plant.
At one spot 8km from the troubled plant, radioactive iodine 80 times the normal level was found.
The government has halted shipments of some foodstuffs in nearby prefectures after the discovery of higher than normal levels of radiation in milk and certain vegetables, but it insists there is no health hazard.
"Food products that present abnormal levels [of radiation] will not appear on the market, so please don't worry. And even if you put such foods in your mouth, they will not have an immediate health risk," said Consumer Affairs Minister Renho, who uses only one name.
Nuclear plant staff and technicians, firefighters and military personnel are struggling to regain control of the overheating facility but spikes in radiation levels have at times forced the crews to suspend work.
The government has declared an exclusion zone with a radius of 20km around the power station and evacuated tens of thousands of people, while telling those within 20 to 30km to stay indoors.
An executive of the under-fire plant operator bowed deeply and apologised at evacuee centres to people forced from their homes by the crisis.
"Since I have tried to manage this problem hand-in-hand with the government, my visit here to directly meet you was belated," said Tepco vice-president Norio Tsuzumi.
"For this I also apologise from the bottom of my heart."
The plant has been hit by a series of blasts since the March 11 tsunami, in the worst nuclear crisis since the Chernobyl disaster in the Soviet Union in 1986.
France's Nuclear Safety Authority warned that local contamination from the Japanese plant would last "for decades and decades".
Workers were forced to evacuate part of the facility on Monday when grey smoke rose from reactor number three, TEPCO said.
Smoke or steam was also seen rising from the number two and number three reactors on Tuesday but work later resumed at the site.
Firefighters again used high-pressure water jets to cool the number three reactor.
A truck with a pump 50 metres high also sprayed water on the spent fuel pool of reactor four, where reduced water levels had led to overheating.
An even bigger truck-mounted concrete pump with a reach of more than 60 metres was on the way from China.
Evacuees endured another night of freezing temperatures, but aid was flowing into the affected areas in greater quantities, according to the UN Office for the Coordination of Humanitarian Affairs.
Some 533,000 people still had no household power and two million people were still without tap water, with 318,213 people huddled in evacuation centres across 16 prefectures, it reported.
But about 90 per cent of the national highways damaged in the twin disaster are now open to the public and more than 90 per cent of disrupted telecommunications have been restored, the UN agency said.
A series of strong quakes measuring above 6.0-magnitude rattled the northeast coast on Tuesday, keeping residents on edge, but there were no reports of damage or injuries.
Tokyo's stockmarket, which took a pummelling for most of last week, jumped 4.36 per cent as the Bank of Japan pumped another Y2 trillion ($24.5 billion) into the money market to calm jittery investors.