A mechanical engineer was jailed for four months on Thursday after he was convicted of commercial sex with an underage escort in the online vice ring case.

Kwa Wee Nam, 34, is the second man after Swiss national Juerg Buergin to have been convicted after trial of having paid sex with the 17-year-old. The former navy engineer was found guilty of paying $450 for the teen's sexual services at Hotel 81 Bencoolen on Oct 8, 2010.

He was among 51 men charged last year in the high-profile case involving the same minor. He is the 24th man to be sentenced so far. Most of the previous offenders who pleaded guilty have been jailed 12 weeks each. Buergin, 42, who was convicted of two charges, has served his four-month and three-week sentence.

Kwa's defence was that he had paid sex with another escort at the hotel, not the minor. The escort, then 35, however, testified that she had never seen Kwa before and that she had no impression of him. Neither did she know the minor nor any of the other escorts working for Tang Boon Thiew, the pimp, who ran the website to market the services of social escorts. The maximum penalty for the offence is seven years' jail and a fine.