Donna Leinwand Leger

USA TODAY

A barrage of artillery fire between North and South Korea across disputed maritime borders on Monday marked an annual show of force by North Korea intent on sending a message to the U.S. as it conducts military exercises nearby.

North Korea's missile launches into the Yellow (West) Sea followed by a threat of live-fire drills along the border "was really aimed at our policymakers, Republic of Korea policymakers and Japan," said Bruce Bechtol, a Korea specialist and professor of political science at Angelo State University in Texas.

"North Korea is saying, 'You can do all the exercises you want and we have the ability to hit you at a moment's notice,'" Bechtol said.

The U.S. and South Korea routinely conduct joint military exercises in the border areas, usually each year in February and March. The most recent exercise began March 27.

The North Koreans said they believe the exercises are meant to intimidate them and often react with some show of force, Bechtol said.

North Korea fired more than 500 rounds of artillery shells over three hours, forcing some residents of South Korean islands to seek shelter in bunkers, South Korean Defense Ministry spokesman Kim Min-seok said.

Last week, North Korea launched Rodong ballistic missiles, which have the range to hit Tokyo or U.S. bases in Okinawa, Japan. Those launches drew condemnation from the United Nations Security Council on Friday.

South Korea responded to the rocket launches on Monday by scrambling F-15K fighter jets and lobbing 300 shells into North Korean waters.

"This is always a dangerous time of year on the peninsula," said Joel Wit, a senior fellow at the U.S. Korea Institute at Johns Hopkins School of Advanced International Studies. "There's a danger here that it may get worse."

Firing rounds into South Korea's territorial water goes beyond North Korea's usual responses. North Korea fired into the water to avoid casualties but still mark its territory and show its willingness to respond with force, Bechtol said.

"This is just short of a violent provocation," Bechtol said. "The intention this time is to stir the pot. "

North Korea, in a statement published by the state news agency, called the drills necessary self-defense "to cope with the grave situation created by the U.S. hostile policy."

The country said it would respond with its own drills, including missiles aimed at "medium and long-rang targets with a variety of striking power," and would consider a fourth nuclear test.

North Korea "is fully ready for next-stage steps which the enemy can hardly imagine in case the U.S. considers them as a 'provocation' again," the statement said. "It would not rule out a new form of nuclear test for bolstering up its nuclear deterrence. The U.S. had better ponder over this and stop acting rashly."

North and South Korea have skirmished over the disputed sea boundary before. In 2010, a torpedo attack sank a South Korean warship, killing 46 sailors. Later that year, North Korean artillery killed four people living on an island.

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