Government prepares big message boards in mind games with North

South Korea is preparing to escalate its psychological warfare against North Korea in response to its latest nuclear test with giant electronic message boards along the border.

South Korea has already set up walls of loudspeakers on the border that have been blaring bursts of anti-North Korean slogans and "K-pop" music since last Friday, a tactic the North considers insulting.

"It takes some time to arrange them because the electronic bulletin boards that were used before were dismantled or old,"Kim Min-seok, spokesman at the South Korean defence ministry, told a news briefing.

But Kim said no final decision had been made on whether to go ahead with the installation.

The bulletin boards in the past carried slogans such as "Come over to the Republic of Korea", South Korea's official name, and "North Korea is a difficult place".

The screens resemble electronic scoreboards at sports stadiums, another military official said.

President Park Geun-hye said on Wednesday the loudspeaker broadcasts against North Korea were "the most effective and certain tool for psychological warfare".

The last time the South used the speakers, in August, it led to a standoff that included an exchange of artillery fire.

Park said the tactic had an impact in the North.

"According to defectors who served on the North's front line they first didn't trust what loudspeakers broadcast, but they came to believe it then crossed the border, risking their lives," Park said in a televised speech.

"The power of truth is the strongest threat to a totalitarian regime," she said.

South Korea's loudspeakers were Seoul's initial retaliation for North Korea's fourth nuclear test, on Wednesday last week, but Park has also called for "bone-numbing" international sanctions.

North Korea appears to have responded with its own psychological operations. It is suspected of sending hot air balloons dropping leaflets critical of the Seoul government into the South on recent nights.

The North is also using its own loudspeaker broadcasts, although South Korean officials say those appear intended to muffle the broadcasts from the South.