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 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 local news media, 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 Hankyoreh newspaper 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 told AFP.

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.

Seoul has now banned this practice.

The North's military supreme command has vowed to take "merciless military retaliatory measures with every means and method involved" until the South stops all such acts and apologises for them.

Professor Yang Moo-Jin of Seoul's University of North Korean Studies said Pyongyang's statements could herald some form of military action.

"North Korea may bring heavy weapons into the Demilitarised Zone," Yang told AFP in reference to the four-kilometre (2.5-mile)-wide buffer zone bisecting the peninsula.

It might also make good on earlier threats to open fire on launch sites for gas-filled balloons which carry anti-Pyongyang leaflets across the border.

Professor Kim Yong-Hyun of Seoul's Dongguk University said the North was resorting to brinkmanship.

But it was restrained by its need for reconciliation with Washington and consideration of the position of its ally China.

"I think the North would find itself in a difficult position to translate threats into actions," he said.