Japanese Prime Minister Naoto Kan said the country was facing its "biggest challenge of the post-war era".

In a televised address, Kan said the Japanese had made their small country a powerful economy and he had faith they would be up to the challenge of rebuilding it, DPA reported.

Technicians were struggling with overheating and reactors at the Fukushima nuclear power plant, where cooling systems were knocked out by the disaster, and explosions have damaged the structures.

Six army vehicles drove up to the stricken plant after sea water dumped from helicopters failed to have the desired effect. The plant was extensively damaged in the 9-magnitude earthquake that triggered a massive tsunami March 11. 

At least 50 tonnes of water was sprayed on the fuel storage pool at reactor No.3, after which steam was seen rising from the damaged building, an indication that the water was reaching the overheating fuel rods, DPA said citing NHK.

In a message to the International Atomic Energy Agency (IAEA) in Vienna, Japan's Nuclear and Industrial Safety Agency Friday upgraded the rating of events at the plant to level 5 on the international 7-step scale, defining it as an "accident with wider consequences".

The previous rating was level 4 - an "accident with local consequences". The Nuclear and Industrial Safety Agency said hourly radiation at No.1 reactor was 279.4 microsievert at 5 a.m. Friday, down from 309 microsievert at the beginning of the crisis and 292 microsievert when helicopters poured water Thursday.

There has been global concern over the crisis at the Fukushima plant, where explosions have taken place at three reactors while a fire engulfed a fourth one.

The storage pools at the power plant lost their cooling function after the quake.
As the crisis deepened, IAEA chief Yukiya Amano met Prime Minister Naoto Kan Friday to discuss the issue.

Amano urged Japan to provide more information on its "extremely serious" crisis, as the battle to regain control of the failing power plant entered the second week.

The UN nuclear watchdog chief said Japan needs to make its information regarding the crisis at the nuclear plant, 240 km north of Tokyo, more readily available to them and the international community, Xinhua reported.

"There is the opinion in the international community that more detailed information is needed," Amano was quoted as telling the Japanese leader.

The government has evacuated residents from a 20-km radius of the Fukushima plant and advised those within a 30-km radius to stay indoors. A no-fly zone has been imposed within 30 km of the plant.

Personnel attempting to cool down the plant have been dubbed a "suicide squad", their families accepting their fate like a death sentence.

The Sankei Shimbun newspaper called the police team "Kesshitai" - meaning a "unit that expects to die", according to Britain's The Independent.

A worker's daughter told Britain's The Sun: "My father says he has accepted his fate much like a death sentence."

Twenty people have been confirmed to have suffered from radiation exposure. About 180 personnel are working round the clock to cool the plant. Working in teams of 50, they enter the radiation hotspots for only 15 minutes at a time to limit their exposure.

The plant's operator Tokyo Electric Power Company (TEPCO) was working to install a new power line to restore electricity to the complex. The new line could enable pumps to send water to the reactors and the pools to keep them cool.

Meanwhile, the toll in the tragedy has been officially been put at 6,539, DPA said. The National Police Agency said 10,259 are officially missing. But local authorities estimate that over 20,000 are missing or unaccounted for.

Life has started limping back to normalcy. But nearly 400,000 remained stranded in over 2,000 shelters, amid shortages of fuel, food and water. 

The bullet train between Morioka, the capital of Iwate prefecture, and Akita, the capital of Akita prefecture, started running again Friday. 

The airport in one of the worst-hit Sendai, capital of Miyagi prefecture, has been opened to emergency services. Damaged ports are also reopening, as well as the Tohoku Expressway, a major road running through the northeast of the country.

But shortages of fuel were still hampering relief efforts, including the distribution of food and other goods. Midwinter level temperature is making matters worse for the affected.