The two Koreas have ramped up their rhetoric since North Korea shelled front-line Yeonpyeong Island near the tense western sea border last month, killing four South Koreans. Both sides accuse each other of provoking first.
On Monday, President Lee Myung-bak used much of his regular address to vow to get tougher with any new provocation by North Korea.
"We have now been awakened to the realization that war can be prevented and peace assured only when such provocations are met with a strong response," Lee said. "Fear of war is never helpful in preventing war."
He said South Korea's military "must respond relentlessly when they come under attack."
South Korea has staged a series of military drills - including one on Yeonpyeong Island on Dec. 20 - in a show of force against the North. The South was to begin routine naval firing exercises starting Monday but not on Yeonpyeong and other border islands, according to the Defense Ministry.
North Korea, for its part, has also kept up rhetoric around last Friday's 19th anniversary of leader Kim Jong Il's appointment as the North's supreme military commander. Kim's military chief threatened last week to launch a "sacred" nuclear war against the South.
On Friday, North Korean soldiers appeared on a state TV program and bragged of participating in the artillery barrage on Yeonpyeong -- the country's first attacks on a civilian area since the 1950-53 Korean War.
The two Koreas are still technically at war because their 1950s conflict ended with an armistice, not a peace treaty. In recent years, several bloody naval skirmishes occurred near their disputed western sea border - drawn by the U.N. at the close of the Korean War.