A 38-year-old army officer was on Friday jailed for five months after paying an underage prostitute $500 for sex.

Emlyn Thomas Thariyan is the 28th man to be sentenced in a series of cases involving the same 17-year-old prostitute, who was part of an online vice ring. He is appealing both conviction and punishment - the stiffest to date. Thariyan committed the offence at Hotel 81 Bencoolen on Feb 18, 2011.

He is the fifth man to be sentenced after claiming trial, following 42-year-old Swiss banker Juerg Buergin, ex-navy engineer Kwa Wee Nam, 34, Indonesian Reyner Desvando Suhartono, 29, and business consultant Edward Whistler Goh Ngian Meng, 51.

Kwa, Suhartono and Goh each received four months' jail, while Buergin, a former executive director with UBS, was understood to have left Singapore in August after completing a 19-week jail term.

A total of 51 men were charged last year. All the 23 other men who have been sentenced had pleaded guilty. The maximum penalty for the offence is a seven-year jail term and a fine.