South Korea will make the Democratic People's Republic of Korea (DPRK) pay for any further provocation as tolerance in the past has only resulted in bigger provocations, South Korean President Lee Myung-bak said Monday.

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"(We) have come to realize it's hard to expect North Korea ( DPRK) to voluntarily give up on its military brinkmanship and nuclear program," Lee said in a nationally televised public speech Monday morning. "South Koreans now know further patience and tolerance will only encourage even bigger provocations," he said. "We will make sure that the North pays for its future provocations. "

The speech came after the two former wartime rivals exchanged fire last week in waters off the divided peninsula near a disputed maritime border. The DPRK shelled a South Korean border island, killing two marines and two civilians.

Lee, who called the artillery bombardment "inhumane," urged national unity and pledged to push for a defense reform designed to shore up the military. The five northernmost islands in the Yellow Sea, deemed vulnerable to the DPRK's future provocations, will also be closely guarded, the president said.

The speech coincided with the ongoing military drills between South Korea and the United States, the latest show of force aimed at deterring Pyongyang's potential aggression. The two allies have said the exercises, planned after the DPRK's alleged torpedo attack in March on a South Korean warship, are "defensive in nature."

Pyongyang, calling its artillery shelling of the island a " defensive measure," warned of a strike against the joint maneuvers. The four-day navy drills, launched on Sunday, mobilize the 97,000- ton aircraft carrier George Washington.