The mall in the town of Kashima was apparently gutted by a gas explosion, a government source told the Kyodo news agency, collapsing ceilings and walls.
Thirteen people died following a major earthquake in Japan on Tuesday, Prime Minister Sanae Takaichi said.
“Thirteen people have died,” Takaichi said Wednesday.
The mall in the town of Kashima was apparently gutted by a gas explosion, a government source told the Kyodo news agency, collapsing ceilings and walls.
“The explosion occurred after the customers and staff had evacuated,” a spokesman for the mall’s operator Aeon told AFP.
Eight people were rescued from the mall, five of whom had injuries, a local official told AFP.
Initial reports said that as many as 20 to 30 people were trapped inside but broadcaster NHK cited police early Wednesday as saying that the number was “lower than that”.
“The exact number is not known. It is certain that staff members are trapped, but it is unclear whether any members of the public are among them,” NHK quoted police as saying.
They added that no safe route of access has been found into the facility, meaning that it has not been possible to enter so far.
At the Nippon Paper Industries factory in Yatsushiro where a smokestack partially collapsed, the local government said that nine people were unaccounted for.
Across Kumamoto — hit by two quakes in as many days in 2016 that killed 273 people — 36,880 households and facilities had no power on Wednesday morning, officials said.
The powerful tremor struck at 4:27 pm (0727 GMT) on Tuesday and was measured at 6.8 by the US Geological Survey (USGS) at a depth of 10 kilometres (6.2 miles).
The temblor sparked several fires, collapsed buildings, damaged bridges — including part of one pedestrian walkway that collapsed into a river — and capsized a freight train.
Chiharu Hara, a 35-year-old recruitment agent who was in Kumamoto, told AFP that she was “terrified” when the quake struck.
“The office swayed violently side to side. I was afraid it would collapse. Everyone panicked,” Hara said.
Footage verified by AFP showed heavy vehicles, street lights and traffic lights swaying. Elsewhere, vehicles on a two-lane road were seen carefully navigating sections of buckled pavement.
Security cameras wobbled violently.
Footage also showed parts of a wall at the centuries-old Kumamoto Castle collapsing.
Many roads were closed and high-speed shinkansen train services were suspended in the region. Kumamoto airport was closed for several hours before re-opening on Tuesday evening.
The Japanese military has mobilised 3,600 personnel for disaster relief operations and sent 20 aircraft to assess the damage from the air, the defence minister said.
Japan is one of the world’s most seismically active countries, sitting on top of four major tectonic plates along the western edge of the Pacific “Ring of Fire”.
The archipelago, home to around 125 million people, typically experiences hundreds of jolts every year and accounts for about 18 percent of the world’s earthquakes.
The vast majority are mild, although the damage they cause varies according to their location and the depth below the Earth’s surface at which they strike.
Japan is haunted by the memory of a massive 9.0-magnitude undersea quake in 2011, which triggered a tsunami that killed or left missing around 18,500 people and wrecked the Fukushima nuclear plant.
