SINGAPORE - A 20-year-old man used Instagram and Facebook to befriend girls aged 12 and 13 - all from single sex schools - before carrying out a string of sexual offences against them.

Goh Kar Aip even threatened to reveal details of one sexual relationship to the girl's school before a teacher got wind of what he was doing and a police report was made.

On Monday (Feb 19) he pleaded guilty to a slew of mostly sexual offences against 10 victims.

The High Court heard that after befriending the girls via private messages, he chatted to them about sex-related topics, before progressing to asking them to send him their nude pictures. He would eventually meet and sexually violate them.

Goh pleaded guilty to four charges - three of sexual penetration of a minor and one of sexual assault by penetration - relating to two of the 10 victims.

Another 34 charges will be taken into consideration when he is sentenced at a later date.

These include one count of criminal intimidation for threatening to locate and tie up a 12-year-old girl when she refused his demands to send him photographs of herself. She was the only one of the 10 who did not fall victim to any sex offences.

Goh, a Singapore permanent resident, was waiting for enlistment in September 2014 when he started using social media to look for young girls. He felt they were "easier to talk to" and would not look down on him for his lack of education, the court heard.

He messaged his targets privately, introducing himself to them as "Kai". While some ignored him, others responded and chatted with him.

After getting to know them, Goh moved on to sexual topics and ask them for photos of their stomachs.

Once he gained their confidence, he would ask for their nude pictures. Sometimes, he would send them pictures of his genitalia in the hope they would reciprocate.

In one case, he asked a 12-year-old to be his girlfriend on their first chat on Instagram. The girl, who had just completed her Primary School Leaving Examination, agreed because Goh said he liked her.

On Dec 19, 2015, Goh took her to his Hougang flat, where he groped her and made her commit an indecent act on him. After this encounter, she wanted to break up with him as she was not comfortable being with someone who much older.

But she eventually agreed to meet him again as she was terrified of his threats to reveal their text conversation about kissing, hugging and masturbation to her school.

Goh later carried out more invasive sexual acts on her at a staircase landing and took photos of himself putting his hand into her clothes.

That same month he began chatting with another 12-year-old, who had sent him several revealing shots of herself.

At their first meeting on Jan 11, 2016 at a staircase landing in Tampines, he fondled her under her uniform and asked her to perform oral sex on him. They met another three times over the next three months, during which Goh committed sexual offences against her.

The offences came to light when a teacher heard that a young man was harassing her students and investigated. The teacher found out that one of the girls who was in a relationship with Goh had taken along three of her schoolmates to meet him.

Goh committed sexual offences against the girl and two of the three friends.

A police report was lodged on March 19, 2016.

A psychiatric assessment on Goh by the Institute of Mental Health found that he did not fulfil the criteria to be diagnosed as a paedophile. However, a psychiatric report said his use of both physical and psychological coercion on the victims likely reflected "an underlying attitude that support or condone sexual violence and would increase his risk of recidivism".

The punishment for the most serious offences, sexual penetration of a minor and sexual assault by penetration, carries a maximum of 20 years' jail and caning .