A couple under police investigation for burying their 5-day-old male infant in secret have been detained over suspicion of murder.

It was the latest in a series of shocking child abuse or infanticide cases revealed after the government launched an investigation to check the well-being of more than 2,000 undocumented babies born since 2015, following the discovery of two unregistered newborns in a home refrigerator last month.

An arrest warrant was issued for a man in his 20s and a woman in her 30s for strangling their son to death and abandoning his corpse in a nearby river in Geoje, South Gyeongsang Province, said the Gyeongnam Provincial Police Agency on Sunday. The Changwon District Court ordered the warrant as there was a high possibility of the couple fleeing.

According to the police, the baby was born in in September last year and discharged from the hospital four days later. The couple initially claimed the baby was found dead the day after it was discharged, and that they had buried the body themselves, as they were financially strapped and could not afford cremation. They said they feared their parents would separate them if they were aware of the pregnancy.

The crime was unveiled after the Goseong-gun government found out the couple had not registered the birth of the infant and reported the child's disappearance to the police last Thursday.

The police dispatched around 80 staff, including riot police, to search the area around the mountain the father had named, but were unable to find the corpse. During further investigation, the father confessed he had strangled the baby and that he had tried to bury the body in the wilderness, but had to change the location due to a number of passersby.

Police are focusing their investigation on finding the infant's corpse around the location the couple said they dumped the body since the date of the alleged crime, with cooperation from the coast guard.