Japan on Saturday scrambled hard to prevent meltdowns at its nuclear power plants, declaring a state of emergency at five atomic reactors and evacuating thousands of residents, as it launched a mammoth relief operation in its Northeast devastated by a massive earthquake that likely left over 1,000 people dead.

"This is the largest earthquake since the Meiji Era (1868-1912), and it is believed that more than 1,000 people have lost their lives," Chief Cabinet Secretary Yukio Edano said at a meeting at the emergency disaster headquarters, a day after the monster 8.9-magnitude tremor struck, unleashing a devastating tsunami.

He expressed his government's determination to bring relief to the disaster-hit areas.

In Fukushima Prefecture, there were reports that radiation 1,000 times above normal was detected in the control room of one nuclear plant, although officials said levels outside its gates were only eight times above normal and asserted that were no health hazards as of now.

The nuclear and industrial safety agency said it issued an unprecedented order for the electricity firm running the atomic unit to open a valve at the plant to release pressure in the container housing the reactor following the powerful earthquake.

The local government, acting on orders from Prime Minister Naoto Kan, instructed about 3,000 residents living within a 10-kilometer radius of the No. 1 nuclear plant in the region and within a 3-kilometer radius of the No. 2 plant to evacuate.

A state of emergency was declared at two reactors at Japan's Daiichi and three units at its nearby Fukushima Daini site, media reports said.

The National Police Agency was quoted by Kyodo as saying that the total number of those died and were unaccounted for in yesterday's catastrophic earthquake topped 1,000, as some areas suffered devastating damage mainly due to tsunami waves of up to 33-foot high.

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.

The Daiichi and Daini power plants operated by Tokyo electric power experienced critical failures of the backup generators needed to power cooling systems after the plants shut down automatically following the quake. Some reports said that about 45,000 people were affected by the evacuation order at the Daiichi plant.

Chief Cabinet Secretary Edano said that there was not any immediate threat to people's health.

"We are taking every possible measure" to prevent disastrous developments, he said, while urging caution in the wake of strong aftershocks that jolted the country this morning.

Several strong aftershocks, one with a magnitude of 6.7 and another of 6.8, rocked eastern coast of Japan, hitting areas like Nagano and Niigata prefectures.

In Iwate Prefecture, the coastal city of Rikuzentakata was almost destroyed by a giant tsunami wave, local police said.

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.

Secretary of state Hillary Clinton offered her condolences in telephone talks with Japanese foreign minister Takeaki Matsumoto this morning and stressed that the US was ready to offer whatever assistance was needed, Kyodo reported.

Premier Kan left Tokyo early this morning by helicopter for Northeastern Japan to inspect the quake-hit areas.