South Korea's president has spoken on national television for the first time since the North Korean artillery attack that killed four people last week.
Lee Myung-bak did not elaborate on how Seoul might respond to any provocation by Pyongyang in the future.
But the BBC reports says he used very firm language in his speech.
Mr Lee described the North's deadly shelling of a South Korean island last Tuesday as an "inhumane crime".
He said he felt deep responsibility for failing to protect residents of Yeonpyeong, where two South Korean marines and two civilians were killed.
Pyongyang says it was provoked by Seoul's military drill near the island.
Tension remains high in the area, where the United States and South Korea are holding joint military exercises.
The North has described the four-day drill in the Yellow Sea, near the disputed border between the two Koreas, as a provocation.
In a seven-minute speech, Mr Lee pledged that Pyongyang would "pay the price in the event of further provocations".
"Attacking civilians militarily is an inhumane crime that is strictly forbidden in a time of war. I cannot help expressing anger at the North Korean regime's brutality."
Tuesday's attack was one of the worst clashes between the two Koreas since the end of the Korean War in 1953.