South Korea's military said Tuesday that three conscripts were found dead in apparent suicides, the latest in a series of incidents fuelling concern over young soldiers serving in isolated frontline units.

A 21-year-old private first class was found shot in his face at the shooting range of an engineer unit some 20 kilometres (12 miles) southeast of Seoul, an army spokesman told AFP.

The soldier, who had been on watch due to mental instability, was believed to have killed himself with his rifle, he said.

Two unnamed corporals, who were both in their early 20s, were found hanged at one of their homes while on leave, the defence ministry said.

Both men were in the 28th Infantry Division serving near the border with North Korea and had been placed on a list of soldiers requiring special monitoring due to concerns over their mental stability, it said.

One of them was deemed unfit to serve last year but continued at the request of his mother, a ministry spokesman said. The soldier had attempted suicide last October, Yonhap news agency said.

Investigations are under way into whether the three soldiers were targets of bullying.

Five soldiers in the same division were arrested last month on manslaughter charges over the death of a 23-year-old private.

An enquiry showed the private had been regularly bullied and physically abused. He died during one beating when he was struck in the chest and a chunk of food became lodged in his airway.

In June, a 22-year-old sergeant opened fire on his barrack mates, killing five and wounding seven.

The sergeant -- also listed as a soldier requiring special observation -- later told investigators that he had been humiliated and constantly mocked.

Two army privates from the same division, also listed as mentally vulnerable, committed suicide later in the same month.

Last week, all military servicemen halted their regular duties to take part in a day-long "moral education workshop" aimed at stamping out bullying.

South Korea's armed forces rely heavily on a compulsory military service system, with conscripts -- most in their early 20s -- accounting for the lion's share of its 690,000 active personnel.

The number of suicides in the military steadily rose from 67 in 2004 to 97 in 2011 before falling to 79 by 2013.

The current rate stands at a little over 11 per 100,000 which, the defence ministry notes, is lower than the national average of 23.5 suicides per 100,000 for South Korean men aged 20 to 29.