Japanese helicopters drop water on nuclear reactor Reuters

Attempts to cool down a stricken reactor at the Fukushima Daiichi nuclear plant in Japan have suffered a further setback with radiation levels rising rather than falling after attempts to douse it with high-pressure hoses.

Six fire engines and a police water cannon were sent in on Thursday evening to spray the plant's No 3 reactor. But afterwards radiation emissions rose from 3,700 microsieverts per hour to 4,000 per hour, the Kyodo news agency quoted Tokyo Electric Power (Tepco) as saying.

An earlier attempt in which military helicopters dropped thousands of litres of water on the plant also appeared to have failed.

As part of the desperate new tactics to avert nuclear meltdown, Chinook helicopters targeted the No 3 reactor's spent fuel rod pool, which is overheating and at risk of releasing dangerous radioactive steam.

Two helicopters flying at less than 300 feet dumped four loads of water. Footage suggested much of it missed the target.

Emergency crews and the military are trying to cool the reactor and replenish a pool containing spent fuel rods. Tepco has been unable to take precise measurements but the pool at No 3 is feared to be almost empty, raising the risk that the rods will overheat and melt, releasing dangerous levels of radiation.

Steam believed to have been caused by water boiling in the pool has been seen rising from it since Wednesday.

Officials are also worried that the No 4 reactor's spent fuel pool might be running low.

'"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 for the government's Nuclear and Industrial Safety Agency who was quoted by the Kyodo agency.

Gregory Jackzo, the chairman of the US nuclear regulatory commission, has told a congressional hearing in Washington that the storage pool at No 4 was in danger of giving off more radioactive material.

Hikaru Kuroda, a Tepco official, said: "We are afraid that the water level at [the No 4 reactor] is the lowest. Because we cannot get near it, the only way to monitor the situation is visually from far away."

Tepco said a military helicopter crew had seen some water in the No 4 pool but this could not be confirmed.

Hydrogen explosions on Monday and Tuesday blew the roofs off the No 3 and No 4 reactors, removing the last line of defence against radiation leaks.

Tepco has said it is attempting to open a temporary power line to the plant so it can pump water directly into the storage pools and reactor cores.

Japan's nuclear safety agency has said it hopes the power supply will be partially operational within hours.

"Once we establish the temporary power supply we will be able to pump seawater into the reactors," a Tepco spokesman said.

"We believe the operation will help cool down the fuel pools," the defence minister, Toshimi Kitazawa, told reporters.

Each helicopter can carry 7.5 tonnes of water per load but the pools each hold 2,000 tonnes, an expert has told public broadcaster NHK.

"It will be possible as long as the rods are fully submerged. That means the storage pool would need to be about a third full. But the dousing has to be done repeatedly."

About 70,000 people have been evacuated from a 12-mile (20km) radius around Fukushima Daiichi. Another 140,000 living outside 12 miles but within 18 miles (30km) have been told to stay indoors.

Japan's cabinet spokesman, Yukio Edano, said there was no need to widen the exclusion zone but signs were emerging that other countries were taking a more cautious approach.

The worsening situation prompted the US to ask citizens living within 80km to evacuate.

''We are recommending, as a precaution, that American citizens who live within 50 miles of the Fukushima nuclear power plant evacuate the area or to take shelter indoors if safe evacuation is not practical,'' the US embassy said in a statement.

The British embassy has since issued similar advice and asked citizens living in Tokyo and northern Japan to consider leaving.

Elevated but not hazardous levels of radiation have been detected well outside the Fukushima evacuation zone. In Ibaraki prefecture to the south, officials said radiation levels were about 300 times normal levels by late Wednesday morning.

It would take three years of constant exposure to these higher levels to raise a person's risk of cancer.