Rescuers are searching for survivors of a huge earthquake and tsunami that have devastated Japan's north-eastern coast, killing at least 1,000 people.
In one town alone - the port of Minamisanriku - 10,000 people were listed as unaccounted for, officials were reported as saying.
Across the region, TV footage showed people stranded on rooftops surrounded by debris-filled water.
Prime Minister Naoto Kan said 50,000 troops were joining the rescue effort.
The government said that all military resources available, including personnel, vehicles, aircraft and vessels, had been mobilised for the rescue effort.
Mr Kan said 3,000 people had been rescued so far.
However, operations are being hampered by aftershocks, continuing tsunami warnings and damaged roads.
An explosion at a nuclear power plant in Fukushima has also prompted fears of a meltdown and people have been ordered to evacuate the area within a 20km (12-mile) radius of the plant.
Cries for help
Hundreds of people are missing, and is it feared that the death toll will rise significantly.
Before-and-after aerial images of coastal towns on Japanese television suggest that virtually entire communities along the coastline were washed away.
The 10m (33ft) tsunami swept up to 10km (6 miles) inland, destroying many towns and villages.
In Minamisanriku, in Miyagi prefecture, reports said the 10,000 people listed as unaccounted-for represented well over half the town's population of 17,000.
NHK World television said the water reached the fourth of a five-floor hospital building.
About a third of the city of Kesennuma, also in Miyagi, with a population of 74,000, was submerged, and the city was also hit by widespread fires.
In Iwate prefecture, Rikuzentakata, a coastal city of some 23,000 people, was almost completely destroyed as the tsunami reached as high as the third floor of the city hall. Some 300-400 bodies were found there.
The coastal area of Miyako and almost all of the town of Yamada, both in Iwate, were also submerged.
A municipal official of the town of Futaba in Fukushima prefecture said: ''More than 90% of the houses in three coastal communities have been washed away by tsunami. Looking from the fourth floor of the town hall, I see no houses standing.''
In one of the worst-hit areas of Fukushima prefecture, people buried under rubble could be heard calling out for help, the Kyodo news agency reported.
Four trains running in a coastal area of Miyagi and Iwate prefectures are still unaccounted for.
In Ibaraki prefecture, north-east of Tokyo, many homes are still without power and there are queues at the few petrol stations that are open, says the BBC's Chris Hogg.
In other developments:
- There have been reports of more than 125 aftershocks, including a 6.8 magnitude quake
- The number of partially or completely destroyed buildings has reached some 3,400
- Some 5.57 million households had lost power as of Saturday morning, while more than one million households have had their water supply cut off, the Kyodo news agency reported
- Fires have continued, with Sendai airport said to still be on fire and Kesennuma in Miyagi Prefecture experiencing three large-scale fires, Kyodo reported
- Two US aircraft carriers are on their way to the disaster zone - the USS George Washington, which is based near Tokyo, and the USS Ronald Reagan, which was on its way to South Korea
- The welfare ministry said 181 welfare facilities, including nursing homes, were damaged
- More than 200,000 people are in emergency shelters. More than 50 countries and territories have offered assistance
The earthquake struck on Friday afternoon off Honshu island, about 400km (250 miles) north-east of Tokyo.
It was nearly 8,000 times stronger than last month's quake in New Zealand that devastated the city of Christchurch, scientists said.