Japan says it is stepping up efforts to cool reactors¹ at the tsunami²-stricken Fukushima Daiichi nuclear plant.

日本政府称，正在想方设法使福岛核电站反应堆冷却下来。

Army helicopters dumped tonnes of water to try to prevent a meltdown of fuel rods. Water cannon³ will join the operation shortly and it is hoped electricity will be restored soon.

Increasing alarm has been expressed in the US at the crisis.

The confirmed death toll⁴ from Friday's 9.0 magnitude quake, which triggered a tsunami, has risen above 5,000.

Police say 5,178 are confirmed dead and another 8,606 people are still missing.

'Deep condolences'

Japan's military CH-47 Chinook helicopters began spraying（喷射） tonnes of water on reactors three and four at Fukushima, 220km (137 miles) from Tokyo, at 0948 local time (0048 GMT), officials said.

The aircraft dumped four loads before leaving the site in order to minimise the crews' exposure to radiation. On Wednesday, the helicopters were forced to abort⁵ a similar operation amid concerns over high radiation levels.

The BBC's Chris Hogg in Tokyo says the helicopters can carry an enormous amount of water but given the high winds it is difficult to know whether it has been dropped accurately⁶.

Japanese government spokesman Yukio Edano said at a news briefing that nuclear experts were now investigating how effective the operation was.

Meanwhile, water trucks are on standby to spray more water on the reactors.

The operation was intended to help cool the reactors and also to replenish⁷（补充） water in a storage pond with spent fuel（乏燃料） rods.

Officials also said they were hoping that later on Thursday they would restore the power supply to the plant, which is needed for the cooling system and backup generators⁸.

"If the restoration work is completed, we will be able to activate⁹ various electric pumps and pour water into reactors and pools for spent nuclear fuel," a spokesman for Tokyo Electric Power Company, which runs the plant, told the AFP news agency.