North Korea has made similar threats without following through. Its latest vow 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.

"Merciless military strike by the Western Front will be put into practice without warning" if South Korean activists make a move to fly leaflets on Monday, the North's military said in a statement in English. It also warned South Korean residents in the border area to evacuate in advance.

In South Korea, Defense Minister Kim Kwan-jin said at a parliamentary hearing that his troops would "thoroughly annihilate" any base responsible for the strike if the North attacked.

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 carrying leaflets criticizing North Korean leaders. North Korea accuses South Korea of supporting the activity, but Seoul denies it.

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.