Workers were temporarily evacuated from the number 2 reactor at the stricken Fukushima Daiichi nuclear plant on Sunday after very high radiation was detected in water, Jiji press agency said.
Japan on Sunday warned that the battle to stabilise a stricken nuclear power plant was far from over, as radiation levels in the sea near the facility rose further.
Plumes of white steam continued to billow from the crippled reactors at Fukushima Daiichi plant, while plant operator Tokyo Electric Power said radioactivity in nearby seawater had surged to 1,850 times higher than normal.
Amid concerns that fuel rod vessels or their valves and pipes are leaking, chief government spokesman Yukio Edano admitted progress at the site was frustratingly slow.
"We'd like to be able to give a clear outline as to when this will be resolved, and those working at the site feel the same way," Edano told public broadcaster NHK on Sunday. "But I can't be any more optimistic than what the reality of it is."
There was also a warning from the head of the world's atomic watchdog agency that Japan's nuclear emergency could go on for weeks, if not months, given the enormous damage to the plant, The New York Times reported.
Japanese authorities were still unsure about whether the reactor cores and spent fuel were covered with the water needed to cool them, Yukiya Amano, director general of the International Atomic Energy Agency, told the newspaper.
Urgent efforts to pump away pools of highly radioactive water near the reactors began Sunday, after several workers suffered radiation burns while installing cables as part of work to restore critical reactor cooling systems.
The incident has heaped yet more pressure on beleaguered TEPCO after the workers, aged in their 20s and 30s, sloshed through highly radioactive water wearing inadequate safety gear.
Edano pledged efforts would be made to report information in a clear and timely manner, amid growing unease over a flow of seemingly erratic and opaque information.
He apologized to residents in the recommended 30 kilometre exclusion zone around the plant over the government's response to the crisis, saying it had not provided them with enough information, Jiji Press reported.
"We will have more detailed monitoring in high-risk areas and increase the capability of making forecasts so as not to be late in tackling this problem," he said.
Slow progress at the Fukushima site has added to the gloom hanging over the country since a 9.0-magnitude quake struck on 11 March, sending a huge tsunami crashing into the northeast coast in Japan's worst post-war disaster.
The wave easily overwhelmed the world's biggest sea defences and erased entire towns.
The confirmed death toll stood at 10,489 Sunday, with 16,621 missing and 2,777 injured, the National Police Agency said.
The tsunami knocked out the cooling systems for the six reactors of the Fukushima plant, leading to suspected partial meltdowns in three of them. Hydrogen explosions and fires have also ripped through the facility.
A worst-case scenario feared at the number-three reactor is that the fuel inside the reactor core - a volatile uranium-plutonium mix - has already started to burn its way through its steel pressure vessel.
Fire engines have hosed huge amounts of seawater onto the plant in a bid to keep the fuel rods inside reactor cores and pools from being exposed to the air, and prevent a full meltdown.
Several hundred metres offshore in the Pacific Ocean, levels of radioactive iodine-131 some 1,850 times the legal limit were reported on Sunday, significantly higher than the 1,250 times increase on Saturday, TEPCO said.
The agency has ruled out an immediate threat to marine life and seafood safety, saying the radiation would be quickly dispersed by tides.
Some progress has nevertheless been made at the site.
The nuclear safety agency on Sunday said workers planned to switch cooling operations to electric pumps from firetrucks at reactor one. High-voltage electric cables have been reconnected to the reactors and power has been partially restored to enable lighting in three reactor control rooms.
Worried about the salt buildup in the crippled plant, engineers have started pumping fresh water into some of the reactors. The US military has supported the effort by sending two full water barges from a naval base near Tokyo.
Radioactive vapour from the plant has contaminated farm produce and dairy products in the region, leading to shipment halts in Japan as well as the United States, European Union, China and a host of other nations.
Singapore extended a ban on food imports from Japan on Saturday, suspending imports of all fruit and vegetables from the whole Kanto region, a large area including greater Tokyo.
Higher than normal radiation had earlier been detected in tap water in and around Tokyo, some 250 kilometres (155 miles) from the plant, leading authorities at one stage to warn against using it for baby milk formula.
Japan has encouraged people living up to 10 kilometres beyond a 20 kilometre exclusion zone around the plant to leave. The 30 kilometre zone is below the 80 kilometres advised by the United States.