SINGAPORE - A 22-year-old man who exposed himself to four girls and insulted the modesty of another in Bishan was jailed for three weeks and fined $2,000 on Thursday.

Full-time national serviceman Kevin Ng Wen Pin admitted to two of five charges.

The court head that on Aug 12 last year, a 19-year-old girl was jogging at Bishan Park when Ng approached her.

He told her he wanted to do a survey and conduct a "social experiment'', and promised not to touch her.

He asked if she had seen a male organ before and if she had sex with her boyfriend. She walked away and contacted the police.

In an earlier incident in August 2013, he was walking behind a 14-year-old girl at Bishan Park when he called out to her. He said he wanted to do a survey but she ignored him.

He persisted and asked if her school covered sex education. He also asked if she had seen a male's private part before, and if her boyfriend was interested in having sex with her. The girl replied "no'', and walked away from him.

Ng overtook her and asked her to sit by the pavement but she declined. He then exposed himself to her. She continued to walk away while he tailed her and started to masturbate.

Assistant Public Prosecutor Asran Samad said it was not a one-off incident and that Ng had targeted five girls, aged 16 to 19, over more than a year in a public park.

Ng said in mitigation there was no excuse for his actions; was remorseful and is working out the problems he has with his psychiatrist.

He could have been jailed for up to three months and/or fined for the obscene act. The maximum penalty for insulting modesty is one year's jail and a fine.