Yasuhisa Shiozaki, the chief cabinet secretary, said the assessment of damage, more severe than expected, was still going on.
"The most important thing is to take necessary measures quickly and respond to the needs of the victims,'' he said.
More than 1,000 policemen and firefighters have been sifting through heaps of debris from destroyed houses searching for survivors.
Japan's defence ministry dispatched 450 troops and seven battleships early on Tuesday to support rescue operations in the region, which has been hit by nearly 100 aftershocks.
Japan lies at the junction of four tectonic plates and is hit by about 20 per cent of the world's most powerful earthquakes.
Authorities were investigating a new possible radioactive leak at a nuclear plant hit by an earthquake in northern Japan, the Kyodo News agency said on Tuesday.
Kensuke Takeuchi, a spokesman at the Kashiwazaki Kariwa nuclear power plant, confirmed that barrels of low-level nuclear waste had tipped over, but did not give further details.
The magnitude 6.8 quake cut off transportation
"We're currently investigating the situation and plan to deal with it as smoothly as possible," he said.
The news came a day after Tokyo Electric Power Co admitted that water containing a "small amount of radioactive material" had leaked from its nuclear plant near the quake epicentre.
Tokyo Electric said some water leaked into the sea of Japan from a reactor that was not in use.
Black smoke also billowed for hours out of the electricity-supplying part of the nuclear facility, believed to be the largest in the world.
Akira Amari, Japan's economy, trade and industry minister, warned Tokyo Electric on Tuesday over the delay in putting out the fire.
"This may cause people to distrust nuclear power," Amari said. "We will not have the plant resume operations without confirming safety."
Tsunehisa Katsumata, the company president, admitted that there was "weakness in our extinguishing measures".
Japan, which has few natural energy resources of its own, relies on nuclear power for nearly 35 per cent of its needs, the second highest figure after France.