The North Korean army has threatened to carry out a "merciless military strike" against South Korea next week in a serious escalation of cross-border tensions.

The Korean People's Army (KPA) vowed to initiate the strike if a group of North Korean defectors now living in the South went ahead with plans to scatter propaganda leaflets on Monday from balloons floated over the border.

"The moment a minor movement for the scattering is captured... a merciless military strike by the western front will be put into practice without warning," the KPA said in a statement carried on the official Korean Central News Agency.

Residents in and around the area where the activists plan to launch their balloons should "evacuate in anticipation of possible damage", the statement said.

"The surrounding area will become targets of direct firing of the KPA," the statement said, adding: "The KPA never makes empty talk."

South Korean defence minister Kim Kwan-Jin vowed to retaliate if North Korea carried through on its threat.

"If that happens, we will strike back at the origin of fire," Mr Kim was quoted as saying by Yonhap news agency.

The defectors plan to carry out the leaflet drop at the border near the town of Paju, around 60 kilometres north of Seoul.

Such exercises are relatively common and North Korea has threatened action in the past, but Friday's statement was unusually strong with its specific naming of the time and location, coupled with the evacuation warning.

It comes at a time of heightened border tensions and a day after South Korean president Lee Myung-Bak made a surprise visit to an island close to the disputed maritime border that was shelled by Pyongyang two years ago.

During his visit, Mr Lee told troops stationed on the island that they should "fight to the death" to protect the border and "retaliate strongly" in the event of any North Korea provocation.

There have been widespread concerns in the South that Pyongyang may try to instigate a military clash that would temporarily destabilise the Korean peninsula in the run up to South Korea's presidential election in December.

AFP