TOKYO (AP) — A powerful earthquake with a preliminary magnitude of 6.4 knocked over houses in southern Japan on Thursday evening, and police said people may be trapped underneath.

There were no immediate reports of casualties from the 9:26 p.m. quake, and no risk of a tsunami.

"There was a ka-boom and the whole house shook violently sideways," Takahiko Morita, a resident of Mashiki, the town at the epicenter, said in a telephone interview with Japanese broadcaster NHK. "Furniture and bookshelves fell down, books were all over the floor."

Morita said some houses and walls collapsed in his neighborhood, and water supply was cut off.

Police in Kumamoto prefecture said they have received reports of a number of collapsed houses and people possibly trapped inside.

Mashiki is east of Kumamoto city, about 1,300 kilometers (800 miles) southwest of Tokyo.

Japan's Chief Cabinet Secretary Yoshihide Suga told a news conference that damage was being assessed, but there were no abnormalities at nearby nuclear facilities.

The epicenter was 120 kilometers (74 mile) northeast of the Sendai nuclear plant, the only one operating in the country.

NHK showed Mashiki town hall in the dark, apparently having lost power. Footage also showed rubble on the road, shards of glasses and broken windows, and fire breaking out in some places, with firefighters battling an orange blaze.

Keisukei Urata, an official at nearby Uki city, said he was driving home when the quake struck at 9:26 p.m.

He also said he saw some walls around houses collapsing.

Parts of the ceiling at Uki City Hall also collapsed, windows were broken and cabinets fell to the ground, he said.

Kasumi Nakamura, an official in the village of Nishihara near the epicenter, said that the rattling started modestly and grew violent, lasting about 30 seconds.

"Papers, files, flower vases and everything fell on the floor," he told NHK. He said there were aftershocks.

One aftershock measuring 5.7 struck about 40 minutes later, according to Japan's Meteorological Agency.

The U.S. Geological Survey put the quake's preliminary magnitude at 6.2 and said it was 23 kilometers (14 miles) deep. It said there's a low likelihood of casualties but some damage is possible.

Footage on NHK showed a signboard hanging from the ceiling at its local bureau violently shaking. File cabinets rattled, books, files and papers rained down to the floor, and one employee appeared to have fallen off a chair, while others slid underneath their desks to protect their heads.