Seoul: South Korea is closely watching North Korea's military after it warned of "merciless" retaliation for anti-Pyongyang slogans displayed by Seoul's frontline troops, the Defence Ministry said on Thursday.

The North's government and military this week lambasted the South's troops for displaying slogans heaping "malignant slanders and calumnies at the army, system and dignity" of the communist state.

Pyongyang late Wednesday revealed details of a warning message it tried to send to the South's presidential Blue House through Panmunjom, the cross-border contact venue, after Seoul refused to accept it.

According to reports, some frontline military units were displaying harshly worded slogans calling for death to the North's leader Kim Jong-Il and his youngest son and heir apparent, Jong-Un.

Defaming the Kims, the object of a Stalinist-style personality cult, is considered blasphemy in the isolated communist state.

The South's newspaper on Monday quoted some other slogans as reading "Let's stick swords and guns into the hearts of North Korean enemy army!" and "A club is the only medicine for a mad dog!"

The Defence Ministry acknowledged slogans were on display at some frontline units but said it had no immediate plan to ban them.

"Following the (Pyongyang) statements, we are now keeping a close watch on the North's movements and our military is fully prepared to counter any threat from it," a ministry spokesman said.

The North said the slogans represented "even more intolerable provocation" after some South Korean military units used the two Kims' portraits as rifle-range targets.