The main reactor at Fukushima nuclear plant in northern Japan, 250 kilometres north of Tokyo, exploded this afternoon leading to radiation leaks injuring four people as the authorities told 45,000 residents living within a 10-km radius of the leaking plant to evacuate their homes, Kyodo news agency reported.

"The plant is experiencing a nuclear meltdown," Japanese nuclear scientists confirmed, after Prime Minister Naoto Kan visited the site in a helicopter early today.

Reports said the main building of the plant housing the reactor as well as the outer walls were blown away in the high intensity explosion which occurred at 3.30 pm local time (1200 IST) as the cooling water levels dropped alarmingly through evaporation after the quake had damaged the main plant.

"The loss of cooling water is leading to a meltdown of the reactor," Jiji Press reported.
TV footage showed smoke billowing from the site and reported that the reactor building had been destroyed. the channels and public broadcasters warned nearby residents to turn off air conditioners and not to drink tap water.

People going outside were told to avoid exposing their skins and to cover faces with masks and wet towels.

Kyodo quoting police said that another nuclear plant adjacent to the one which exploded was also malfunctioning with the radiation level reaching almost 1000 times the normal level.

The agency said the damage caused by Friday's catastrophic earthquake in Japan expanded today, with the combined number of people who died or are unaccounted for feared to top 1,600.

The Tokyo fire department send specialised "hyper rescue teams" to the exploded nuclear plants.

The authorities said that an emergency had been declared at all five nuclear reactors as the units had lost cooling ability, as US dispatched plane loads of coolants to the disaster struck nation.

They said that Fukushima, Daiichi plant unit no. 1 had exploded after failing to tamp down heat and pressure inside the reactor which had suffered extensive damaged due to tidal waves set off by the earth quake which cut down the power supply to the plant.

Japan's Nuclear Safety Agency described the situation at the Fukushima plant dire. Defence Ministry officials said dozens of troops and fireman trained for chemical disasters had been dispatched to the plant, which is located South of the Miyagi Prefecture which was hardest hit by the quake.

As Japan is one of the most seismically active country in the world, the government met has strict set of regulations designed to limit the impact of quakes on nuclear plants. But even so 10 of the Japan's 54 commercial reactors were shut down because of the quake.
Meanwhile, four trains running in a coastal area of Miyagi and Iwate prefectures remained unaccounted for, the train operator said.

It is not known how many people were aboard the trains that were running on East Japan Railway Co.'s Ofunato, Senseki and Kesennuma lines on the Pacific coast when the quake hit northern Japan.

The company said earlier that another train on the Senseki Line was found derailed near Nobiru Station after the quake. The Miyagi prefectural police today rescued nine passengers from the train by helicopter, Kyodo said.

The number of partially or completely destroyed buildings reached 3,400, while there 200 incidents of fire at quake-affected areas. Some 181 welfare facilities, including nursing homes, had been damaged.

Around 200 to 300 bodies were found in Sendai's Wakabayashi Ward, they said. Some 1,800 houses in Fukushima Prefecture were found to have been destroyed.

As rescuers have not been able to completely access the tsunami-hit areas with tsunami warnings still in effect, the overall picture of the destruction remained unclear.

A municipal official of the town of Futaba, Fukushima, said, "More than 90 per cent 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 the quake-hit areas, around 5.57 million households had lost power, while 600,000 had their water supply cut off.

Nine expressways were closed and at least 312 domestic flights cancelled. The Tokyo police said more than 120,000 people in the capital were unable to return home last evening due to the suspension of train operations and traffic jams.

Rescue teams from South Korea, Australia, New Zealand, Singapore and other countries were set to arrive in Japan, after 50 nations offered support following the powerful earthquake, the Japanese Foreign Ministry said.

In Washington US President Barack Obama called Japanese Prime Minister Kan to offer help.

"(First Lady) Michelle (Obama) and I send our deepest condolences to the people of Japan, particularly those who have lost loved ones in the earthquake and tsunamis," Obama said in a statement.