SINGAPORE - Two half-sisters treated their mother's boyfriend as their stepfather - but the 29-year-old man abused their trust by molesting them as they slept.

Between April 17 and May 8 last year, the Malaysian man molested the eight-year-old once and the 13-year-old six times while their mother and two other siblings were asleep in the same room

The man, who cannot be named due to a gag order to protect his victims' identities, pleaded guilty on Monday (May 7) to three counts of molestation involving both girls.

Four other outrage of modesty charges involving his older victim will be taken into consideration during sentencing.

The girls have different biological fathers and their mother has custody of them.

Deputy Public Prosecutor Michael Quilindo said she met the man in 2014 and the unmarried couple had a daughter together the following year.

He began living with his girlfriend, her father and her children - also including two sons - last year. Her father slept in a bedroom while the rest slept in the living room.

The court heard that on one occasion he touched the eight-year-old's private parts as she slept, stopping when she stirred. He did the same to the older girl under similar circumstances.

DPP Quilindo told District Judge Kessler Soh: "During investigations, the accused admitted that he would only sexually touch the victims when it was dark at night and everyone was sleeping. When asked why he had performed such acts against them, he said ' It was an urge I guess. I can't control my sexual urge'."

An Institute of Mental Health report said he has paedophilic tendencies which appeared to have remained dormant until the offences occurred.

DPP Quilindo said: "Although the accused claimed that the offending behaviour was opportunistic in nature and blamed the victims' mother for forcing him to sleep next to the victims, it would appear that he made no attempt to remove himself from the situation, nor insisted that the girls do not sleep next to him."

The cases came to light when the eight-year-old girl's child protection officer from the Ministry of Social and Family Development made a police report on May 19 last year.

Court documents did not reveal details of how the officer found out about the girl's plight.

The offences involving the older girl were revealed during investigations.

He is expected to be sentenced on May 14. For each count of molesting a girl under 14, he can be jailed for up to five years and fined or caned.