A 29-year-old man was being sought by police after his 12-year-old niece was beaten to death and her body buried in a box beneath the kitchen floor of their home in Bangkok.

Police dug up the large plastic box from the dirt beneath the floor of the downstairs kitchen of the two-storey house in Bang Khen district about 12.30am on Tuesday.

The body was naked and wrapped in black garbage bags and towels. There were many severe impact wounds to the face and body. A doctor estimated the girl had been dead about 48 hours, police said.

A woman who identified herself only as Apinya called police to the scene. She said the girl was a distant relative.

The girl had lived with her aunt, who was Apinya's cousin, and with the aunt's husband.

According to Ms Apinya, the girl was aggressive and often stole things at home. The aunt and her husband frequently scolded the girl, and also hit her.

Past punishment had only made the girl more aggressive and the couple had told Ms Apinya they did not know how to deal with her, according to police.

About 7pm on Monday the couple visited Ms Apinya. The man told her he had beaten the girl to death with a baseball bat about 1am on Sunday.

He said he did not intend to kill the girl, wanting only to punish her for stealing.

The man told her that on Sunday evening he had bought a large plastic box, put the body inside and buried it, covered over with soil and concrete, Ms Apinya told police.

He then asked Ms Apinya to call the police, and he left. Ms Apinya said she thought the man went to see a friend in Din Daeng district, but had been unable to contact him.

Police held his wife for questioning.