At least two people died and more than 400 were injured after a 6.5-magnitude earthquake struck Japan's southern island of Kyushu on Thursday.
The quake struck at 9:26 pm (1226 GMT) at a depth of 11 kilometres, with its epicentre in the Kumamoto region, but no tsunami warning was issued, the Japan Meteorological Agency said.
By early Friday, more than four hours after the quake, the meteorological agency reported 20 aftershocks and it also upgraded the initial tremblor from a preliminary magnitude of 6.4.
It said more aftershocks were expected in the coming days.
The quake collapsed walls and a number of houses in the town of Mashiki, 900 kilometres south-west of Tokyo. At least one person was killed, broadcaster NHK reported, adding that several people could still be trapped.
Another person died in a fire triggered by the quake, the report said.
Japanese Prime Minister Shinzo Abe told reporters he had discussed the situation with Kumamoto Governor Ikuo Kabashima over the phone.
"I told him the government will help [the affected region] with utmost effort," Abe said.
The military has dispatched aircraft and helicopters to the quake-struck region to assess the extent of damage, Defence Minister Gen Nakatani told a news conference.
Chief Cabinet Secretary Yoshihide Suga urged those affected to remain calm and to help each other.
He also said there were no abnormalities reported at the Genkai Nuclear Power Plant in Saga prefecture or the Sendai Nuclear Power Station in Kagoshima prefecture.
The Sendai plant is 130 kilometres south of Kumamoto. The operator restarted two nuclear reactors last year, the first of two units under updated regulations.
The quake prompted the suspension of train services in the region, according to NHK.
An agency official said Thursday's quake was the first recorded at level 7 on the Japanese seismic scale since the earthquake and resulting tsunami in the north-east on March 11, 2011.
Those twin natural disasters killed about 18,500 people and triggered meltdowns at the Fukushima Daiichi Nuclear Power Station.