Scores of people were feared to be buried alive after a magnitude 7.0 earthquake struck southern Japan early on Saturday, killing at least 18, and sparking collapses and fires, barely 24 hours after the first disaster struck.
Authorities warned of damage over a wide area, as reports came in of scores of people trapped in collapsed buildings, fires and power outages.
"We are aware of multiple locations where people have been buried alive," Chief Cabinet Secretary Yoshihide Suga told a press conference.
"Police, firefighters and Self Defense Force personnel are doing all they can to rescue them."
Saturday's tremblor triggered a tsunami advisory, although it was later lifted and no irregularities were reported at three nuclear power plants in the area, said a senior government official.
A fire erupted in what appeared to be an apartment building in Yatsushiro city, while some people were trapped in a nursing home in the town of Mashiki, according to NHK.
There were no irregularities at Kyushu's Sendai nuclear plant, which continued to operate, or at the Genkai plant, also on Kyushu, or the Ikata plant on nearby Shikoku, according to NHK.
Chief Cabinet Secretary Yoshihide Suga said nearly 80 people were believed to be trapped or buried in rubble. Extra troops would be sent to help, with up to 15,000 due on Saturday, as well as more police, firefighters and medics, he said.
"We are making every effort to respond," Suga said.
Troops fanned out to search ruined houses as dawn broke.
The epicentre of the quake was near the city of Kumamoto and measured at a shallow depth of 10 km, said the US Geological Survey. The entire city of 730,000 was without power.
Prime Minister Shinzo Abe, arriving at his office, told reporters the government was making every effort to determine the extent of the damage, carry out rescue and recovery, and to get accurate information to citizens.
"Nothing is more important than human life and it's a race against time," Abe said at a government meeting after the disaster. "I want rescue activities to continue with the utmost effort."
The region's transport network suffered considerable damage with one tunnel caved in, a highway bridge damaged, roads blocked by landslips and train services halted, local media reported.
On Thursday a magnitude 6.0 earthquake struck southern Japan leaving nine people dead.
People still reeling from the after shock, poured onto the streets after the Saturday quake struck at 1:25 am. (1625 GMT).
"Thursday's quake might have been a foreshock of this one," Shinji Toda, a professor at Tohoku University, told national broadcaster NHK.
A magnitude 9 quake in March 2011, to the north of Tokyo, touched off a massive tsunami and nuclear meltdowns at Fukushima. Nearly 20,000 people were killed in the tsunami.