Unable to convince his girlfriend to have sex with him more than once a month, Alistair Pow Jun Hao identified a friend's contact on Facebook to help make up the difference.

He befriended the girl, then 13, on the social networking site and propositioned her while the two were watching a movie together at his home last year - trying to persuade her by getting his girlfriend to pose over the phone as a male friend threatening to break off his "friendship" with Pow if the victim refused.

Pow, 19, was sentenced on Monday to 24 months' probation by a district court and ordered to perform 150 hours of community service, after pleading guilty to two counts of sexually penetrating the minor and another young girl.

Pow and his girlfriend, according to court papers, would only have intercourse once a month as she would reject his frequent requests for it.

This led Pow to "add" his 13-year-old victim on Facebook after finding her through a mutual friend. He invited her to his flat in February 2013 to "chat as well as watch movies", where she later refused his request for sex.

He put his girlfriend on the phone, where she told the 13-year-old in a "coarse" voice she would stop being friends with Pow if she refused. Pow then said the friendship would "sour" and this would affect their relationship too. The minor said she would "think about it" but eventually relented, asking him afterwards if he loved her. Pow nodded and said he would not leave her.

Pow's other sexual offences with his previous girlfriend, then 14, took place between February and April 2012 when he was serving his full-time national service.

The maximum penalty for sexual penetration of a minor aged under 16 years is a jail term of 10 years and a fine.