North Korea has threatened to attack a South Korean border area where anti-Pyongyang activists plan to drop leaflets from balloons.
North Korea has made similar threats in the past without following through. The vow came a day after South Korea's president warned against provocation as he visited a frontline island shelled by North Korea in 2010.
South Korea's defence minister said his troops will retaliate if the North attacks.
South Korean activists regularly send up balloons carrying leaflets criticising North Korean leaders.
The North said it will "strike mercilessly" if leaflets are flown as scheduled on Monday. It also warned South Korean residents to evacuate.
The 2010 shelling killed four South Koreans.
Seoul also blames Pyongyang for the deadly sinking of a warship that year. North Korea denies the claim.