North Korea's military has threatened to strike a South Korean border area where anti-Pyongyang activists plan to launch leaflets from balloons next week. South Korea immediately vowed to retaliate if attacked.

North Korea has made similar threats without following through. Its vow on Friday came a day after South Korean President Lee Myung-bak warned against provocation as he made a surprise visit to a front-line island shelled by North Korea in 2010.

The North's National Defense Commission lashed out at Lee on Saturday, accusing him of trying to shore up political support for his conservative camp ahead of presidential elections in December.

"Merciless military strike by the Western Front will be put into practice without warning" if South Korean activists go ahead with plans to launch leaflets criticizing North Korean leaders on Monday, the North's military said in a statement in English on Friday. It warned South Korean residents in the border area to evacuate in advance.

In South Korea, Defense Minister Kim Kwan-jin told a parliamentary hearing that his troops would "thoroughly annihilate" the responsible base if the North attacks.

The exchange of strong warnings came as Glyn Davies, the top U.S. envoy for North Korea, met in Seoul with Lim Sung-nam, South Korea's envoy to stalled six-nation talks on North Korea's nuclear arms programs.

Davies did not comment on the North's threat during a meeting with reporters, but urged Pyongyang to follow through with its commitments made in past nuclear agreements with the United States, South Korea, Russia, China and Japan.

North Korean defectors and South Korean activists regularly send up balloons to drop propaganda leaflets in North Korean territory. North Korea accuses the South Korean government of supporting the activity, but Seoul denies it.

Despite the North Korean threat, activists will go ahead and launch around 200,000 leaflets from the border area of Imjingak as planned, North Korean defector Park Sang-hak said Saturday.

Animosity has run high between the Koreas since the North's 2010 shelling killed two marines and two civilians on South Korea's Yeonpyeong island in the Yellow Sea. Seoul also blames Pyongyang for the sinking of a warship that killed 46 South Korean sailors earlier that year. North Korea denies attacking the ship.

The two Koreas remain technically at war after the 1950-53 Korean War ended in a truce, not a peace treaty.