Japanese engineers have connected a high-power line to the critically damaged Fukushima Dai-ichi nuclear plant's Unit 2 reactor, a UN nuclear agency reports.

If power is successfully restored, engineers may be able to restart pumps to send a steady supply of water to cool down fuel cells.

The Unit 2 reactor has taken precedence over two other troubled reactors, because the roof of the building remains intact. Explosions at the other two buildings left the interior exposed, allowing water to be dropped from helicopters or water cannons.

Power could be restored Friday or Saturday, officials said.

Earlier, emergency workers continued last-ditch efforts to curb the nuclear disaster unfolding at Japan's tsunami-struck power plant, as the International Atomic Energy Agency said the situation remained "very serious" but stable.

Nearly a week after a 9.0-magnitude earthquake and tsunami damaged the Fukushima Dai-ichi plant and devastated the country's eastern coast, workers tried to gain control over the situation.

It included dropping water by helicopter, blasting it from fire trucks and using a police water cannon in a desperate attempt to cool the damaged reactors.

Tokyo Electric Power Co, which operates the facility, said the operation has proven to be somewhat successful at the Unit 3 reactor, Japan's Kyodo news reports.

Meanwhile, an official at the IAEA said the situation at the plant remained "very serious" but it did not appear to have deteriorated over the past 24 hours.

"It is still possible that it could get worse," said Graham Andrew, a senior aide to International Atomic Energy Agency chief Yukiya Amano.

Andrew's comments came after Amano flew to Tokyo to gauge efforts to combat the nuclear emergency let loose by Friday's twin disasters.

Four of the six reactors at the Fukushima Da-ichi plant have faced serious crises involving fires, explosions, damage to the structures housing reactor cores, partial meltdowns or rising temperatures in the pools used to store spent nuclear fuel.

Officials have also said temperatures are rising in the spent fuel pools of the last two reactors. As a result, they are desperate to cool the reactors and pools, which could release highly dangerous radioactive materials.

"The highest priority now is to pour adequate water onto the No. 3 and No. 4 reactors, especially in their spent fuel pools," said Hidehiko Nishiyama, a spokesman of the government's Nuclear and Industrial Safety Agency.

Crews aboard a pair of Japanese military helicopters began braving the radiation to fly over the plant's six side-by-side nuclear reactors Thursday morning local time, dumping thousands of litres of sea water on the critically damaged Unit 3.

"We decided to do this because we thought that today is the time limit," said Defense Minister Toshimi Kitazawa, in Tokyo.

The water was also aimed at replenishing the rooftop pool that houses the reactor's spent nuclear fuel.

Kitzawa said he believes the water from the helicopters reached the reactor. However, plant operator Tokyo Electric Power Co. said the radiation level was the same before and after the air drops.

Unlike active fuel rods which are protected by thick steel containers, the spent fuel rods are more susceptible to exposure to the environment. As the water disappears and they come in contact with the air, metal cladding on the spent fuel rods can catch fire and release a toxic combination of radioactive elements.

Helicopter mission

In a desperate bid to prevent that from happening, crews flying CH-47 Chinook helicopters alternated flights, limiting their exposure during the four flyovers of their 12-minute mission. However, strong winds appeared to blow most of their 7,500 litre payload off target.

"It appears as if much of the water has been swept up in the air," CTV's Tom Walters said in a telephone interview from Osaka, Japan.

"It is a kind of Hail Mary pass to get water into that cooling pond and bring down the temperature there," he said.

In a news conference, top government spokesperson Yukio Edano said special police units were called to use the same high-powered water cannons typically deployed against rioters. Japan's Kyodo News service reports that one police water cannon truck sprayed briefly before its work was suspended.

Instead, six military firefighting vehicles designed to douse burning plane wrecks sprayed water onto the No. 3 reactor.

Japan's Defence Ministry could not say whether the water reached the storage pool containing spent fuel rods, broadcaster NHK reported. But the ministry was preparing to repeat the exercise on Friday if the government issues a new request.

Meanwhile, at the Unit 4 reactor, there are concerns that water has completely evaporated from the cooling pool for its spent fuel rods.

"There is no water in the spent fuel pool and we believe that radiation levels are extremely high, which could possibly impact the ability to take corrective measures," chief of the U.S. Nuclear Regulatory Commission Gregory Jaczko said at a House Energy and Commerce subcommittee hearing Wednesday.

On Thursday, plant operator Tokyo Electric Power Co. said they believed the rods are still covered, but in subsequent comments Yuichi Sato of Japan's nuclear safety agency edged closer to Jaczko's view.

"Considering the amount of radiation released in the area, the fuel rods are more likely to be exposed than to be covered," Sato said.

But after conceding its airborne water dumps had done little to reduce the temperatures at Unit 3, the military said it had postponed plans to drop water on Reactor No. 4.

Japanese police estimate more than 452,000 people have been displaced by the quake that struck off the country's northeastern on March 11, and by the ensuing tsunami.

The official death toll is now 5,692 as of Friday morning, with 9,522 people missing, according to police.

Approximately 70,000 people have been evacuated from the 20-kilometre exclusion zone surrounding the Dai-ichi plant in Fukushima, while another 140,000 residents of the area between 20- and 30-kilometres from the plant remain under instruction to stay indoors.

With files from The Associated Press