A man has allegedly confessed to police that he murdered his fiancee just five days before their planned wedding ceremony.

Ronnachai Panchart admitted to killing Nontiya Krowjatturat in the residential compound of the Tambon Salui Health Promotion Hospital in Tha Sae district in the early hours of Tuesday, Chumphon provincial police chief Pol Maj Gen Sonthichai Awattanakulthep said on Wednesday.

He confessed after being detained and interrogated throughout the night.

The marriage of Mr Ronnachai, 26, to Ms Nontiya, 25, a dental official at the community health office, was scheduled for this coming Sunday at a school in Sawi, Nontiya's home district.

Mr Ronnachai is an employee of the Provincial Electricity Authority in Lang Suan district.

The motive for the murder was unclear, but there seemed to be another woman in the story, investigators said.

The couple's wedding plans reportedly soured when the bride-to-be went to Mr Ronnachai's house and found him with another woman. This lead to a quarrel. Nontiya had previously been unaware of her fiance's behaviour.

Her father Mr Chao said on Tuesday that Nontiya told him she had problems with her future husband, but did not go into details.

Nontiya was killed by a shotgun blast at around 1am on Tuesday. Her body was found by colleagues from the hospital after she failed to report for duty later that morning. They went to her quarters and broke in through the locked door after noticing blood stains. They quickly called police.

Investigators said the direction that the security camera outside her room was pointing had been changed and the TV inside the room was still on.

Police watched footage from other cameras and saw a man concealing his face with a mask enter the compound at around 1am and leave about 20 minutes later.

Police searched Mr Ronnachai's house in Lang Suan later on Tuesday and, according to Pol Maj Gen Sonthichai, they found clothes similar to those worn by the man in the camera footage.

Police had not yet determined whether the other woman was involved in the crime.