Japanese military helicopters have dumped water on an overheating nuclear plant damaged by last week's deadly earthquake.
Cooling functions at the Fukushima Daiichi plant, about 220km north of Tokyo, have been damaged and workers are engaged in a desperate bid to avert a nuclear meltdown.
On Thursday morning, Chinook helicopters dumped around 30 tonnes of seawater, all aimed at two of the plant's six reactors - units 3 and 4.
Al Jazeera's Andrew Thomas, reporting from Osaka, said "four flights were made and of those water-drops by the helicopters, only one hit its target". The effort has since been abandoned, he said.
"Given that more than a hundred would be required to hit dead-on to fill the reactors with enough water to cool the rods, it is – forgive the phrase – a bit of drop in the ocean at the moment," our correspondent said.
The cooling systems at both reactors are not functioning, raising fears that spent fuel rods could melt and release radioactive material outside the building.
Police water cannons were also set to support the effort in addition to equipment already in use over recent days at the plant, while military vehicles designed to extinguish fires at plane crash sites were also deployed.
Toshimi Kitazawa, Japan's defence minister, said pumps supplied by the US armed forces were also being transferred.
Tokyo Electric Power Co, which operates the plant, said it will re-attempt on Friday to connect outside power cables to two of the plant's reactors in an effort to restart their cooling pumps, after a first attempt was unsuccessful.
The number 2 reactor is said to be the first to receive electricity.
But as authorities scrambled with a patchwork of fixes, the top US nuclear regulator warned that the cooling pool for spent fuel rods at reactor number 4 may have run dry and another was leaking.
Gregory Jaczko, head of the Nuclear Regulatory Commission, told a parliamentary hearing that radiation levels around the cooling pool were extremely high, posing deadly risks for workers toiling in the wreckage of the earthquake-shattered power plant.
"It would be very difficult for emergency workers to get near the reactors. The doses they could experience would potentially be lethal doses in a very short period of time," he said in Washington. Japan's nuclear agency said it could not confirm if water was covering the fuel rods.
The government's Nuclear and Industrial Safety Agency said the top priority should be pouring water into the fuel-rod pools at reactors 3 and 4, which may be boiling and are not fully covered by roofs that would reduce radiation leaks.
The nuclear agency said two radioactive substances, cesium and radioactive iodine, have been detected near the Fukushima number 1 reactor.
The agency said this indicates that some of the metal containers of uranium fuel may have started melting. The substances are produced by fuel fission, NHK, Japan's broadcaster, said.
Bad weather conditions are impeding rescue and relief efforts [Getty]
Naoto Sekimura, a University of Tokyo professor, told NHK that only a small part of the fuel may have melted and leaked outside.
He called on residents near the power station to stay calm, saying that most of the fuel remains inside the reactor, which has stopped operation and is being cooled.
Some 70 workers have been using pumps to pour seawater to cool reactors at the plant, according to media reports, using electricity from borrowed mobile generators.
Paul Carroll, a programme director at Ploughshares, an international nuclear security foundation, told Al Jazeera that the engineers at the plant are doing heroic work.
"In order to be adequately protected from the radiation they would need to have essentially leaded shielding. If that is what they are equipped with, it would make it extremely difficult for them to actually move around.
"I suspect that these are almost – I hate to say it – suicide missions. These workers have signed up for a mission that puts themselves behind their countrymen," Carroll said.
Imad Khadduri, a nuclear scientist based in Qatar, told Al Jazeera that Japanese authorities are "thinking of bringing in retired workers to the plant because they have a short lifetime left".
"People who are exposed to such high levels of radiation can only do that for a certain period before developing radiation sickness and will have to be replaced," Khadduri added.
Meanwhile, the official death toll from last Friday's twin disasters has risen to 5,198, Japanese police has confirmed, with relief efforts being hampered by adverse weather conditions in the north of Japan.
The 8.9-magnitude quake - the biggest in Japan's history - triggered a massive tsunami that decimated large tracts of the country's northeastern coastline.
"Half a million people are still living in evacuation centres. The cold and the ice make rescue efforts very difficult," our correspondent said.
The US state department late on Wednesday authorised the voluntary departure of embassy family members in quake-damaged Japan.
"We have not ordered them to leave. We have made this opportunity available to them should they choose to exercise it," Patrick Kennedy, a state department official said in a conference call to reporters.
The authorisation applies to around 600 family members of diplomats in the US embassy in Tokyo, the consulate in Nagoya and a language school in Yokohama, Kennedy said.