http://www.bbc.co.uk/news/world-asia-pacific-12781784Japan nuclear plant progress slowEmergency workers are battling to cool and restore power to reactors at the earthquake-damaged Fukushima Daiichi nuclear plant but say progress is slow.
Power is needed to restart pumps to pour cold water on the overheating fuel rods and avert a major radiation leak.
There have been a number of explosions in the reactor buildings since Friday's magnitude 9.0 earthquake in Japan.
More than 16,000 people are either confirmed dead or listed as missing after the quake and subsequent tsunami.
Japanese police say 6,405 people are known to have died and around 10,200 others are missing.
But Kyodo news agency reports that the official toll is based on names registered with police, and that the true figure could be in the tens of thousands.
Across the country, people observed a minute of silence at 1446 local time (0546GMT) on Friday, exactly a week after the huge quake.
Live television showed elderly survivors in evacuation centres bowing their heads in respect for the dead.
Race against timeThe atomic crisis was triggered when the power supply to Fukushima, about 240km (150 miles) north of Tokyo, was damaged by the natural disaster and back-up generators failed.
Smoke was reported to be billowing from the plant on Friday morning and it was not clear what was causing it.
At a press conference, Japan's Chief Cabinet Secretary Yukio Edano spoke of steam emitting from the plant after helicopters had sprayed water over the affected reactors on Thursday.
Fire trucks are now being mobilised from as far as Tokyo in a bid to hose the reactors down.
Television pictures showed water jets being turned on the reactors.
Officials say they are also hoping to fix electricity cables from the grid to at least two of the six reactors on Friday.
Power may then be restored to reactor 2 as early as Friday night and to reactors 3 and 4 possibly by Sunday, Japanese news agencies reported.
Even if the engineers manage to connect the power, it is not certain that the pumps will work as they may have been damaged in the earthquake or subsequent explosions."Preparatory work has so far not progressed as fast as we had hoped," an official of plant operator Tokyo Electric Power Co (Tepco) told a news briefing.
Workers there are being constantly checked for radiation levels, he told reporters.
Mr Edano insisted radiation levels had not been serious enough to affect human health, although he conceded that there had been some higher readings.
He said reactor 3 remained the greatest concern and would be sprayed continuously.
Mr Edano also denied reports that Japan had rejected US offers of help.
"We have repeatedly asked for specific support, and indeed, they are responding to that," he told reporters.
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US Nuclear Regulatory Commission Chairman Gregory Jazcko has said that it could take days and "possibly weeks" to get the complex under control.
The head of the International Atomic Energy Agency (IAEA) flew into Tokyo on Friday with a team of four experts to be briefed by Japanese officials.
Yukiya Amano, a Japanese citizen, said he views Japan's nuclear crisis as an extremely serious accident requiring international co-operation.
Humanitarian crisis
Japan has imposed a 20-km (12-mile) exclusion zone around Fukushima and has urged people living up to 30km away to stay indoors.
Some countries have advised their nationals in Japan to stay up to 50km away or to leave the country entirely.
Many of those who have been evacuated from the exclusion zone are staying in temporary shelters.
Police said more than 450,000 people have been made homeless by the disasters. Some are sleeping on the floors of school halls.
Bitter winter weather has added to their misery.
Even further away from the immediate quake zone, tens of thousands of homes are said to be without electricity and the government estimates more than 1.6m households have no running water.
Aid workers have appealed for more help as supplies of fuel, medicine and other necessities are running short.
The European Union's humanitarian aid commissioner Kristalina Georgieva said the EU stood "ready to provide any help" in the face of requests for blankets, mattresses, water, water tanks, food and tents.
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