A South Korean soldier accused of killing five of his comrades opened fire because he was fed up of cartoons likening him to SpongeBob SquarePants, military officials said.

The man (22) known only by his surname - Yim - allegedly shot members of his own unit near the border with North Korea last month. He was captured two days later after shooting himself in the chest in an apparent failed suicide attempt.

Brigadier General Seon Jong-Chul, a senior military investigator, said Yim had been picked on by both lower and higher-ranking members of his unit.

On the day of the attack, cartoons were drawn on the back pages of a logbook, depicting Yim as a number of cartoon characters, including SpongeBob, a yellow sponge who lives in a pineapple under the sea with his pet snail.

Mr. Seon said the cartoons "triggered a flashback" to when Yim was bullied in high school, adding "Other troops saw it as a small joke but he said he had been greatly distressed by it".

Before shooting himself, Yim wrote a brief note saying "life is more painful and difficult than death".

"Like little children who kill insects without feeling guilty, they do not know how much pain they can give to someone" he added.

Yim, who joined the army in 2012 and was three months from being discharged, had difficulty adapting to military life, it has been reported.

Six survivors of last month's shooting who allegedly took part in the bullying are expected to face disciplinary action.

A spokesman for South Korea's defence ministry said prosecutors plan to charge Yim, who has started legal action against one of his senior officers.