North and South Korea Exchange Fire Across Disputed Sea Border

SEOUL, South Korea — North Korea conducted extensive live-fire military drills off its southern coast on Monday, some of its artillery shells falling south of the disputed sea border with South Korea, in a military provocation that came a day after the North threatened to conduct more nuclear tests.

Marines in South Korean border islands fired back, launching artillery shells north of the disputed sea border, the South Korean military said.

South Korean officials said on Monday that the shells from both sides appeared to have fallen harmlessly into waters from which naval and fishing boats have stayed clear.

Such exchanges of artillery in the disputed waters were not unprecedented, but rising military tensions there indicated that after months of a relative lull, hostilities between the two Koreas have begun ratcheting up again. And they raised fears that the often-repeated cycle of peace overtures and provocations turned its wheels once again on the divided Korean Peninsula.

Citing the joint military exercises Washington and Seoul started in late February as a justification, North Korea has test-fired a series of rockets and short- and midrange ballistic missiles in recent weeks.

The tests prompted the United Nations Security Council to warn last week of more censure against the country, which is already under heavy sanctions.

On Sunday, Pyongyang lashed out by threatening “a new form of nuclear test” and warning that its Korean People’s Army would conduct drills aimed at improving its ability to attack mid- and long-range targets with “more diversified nuclear deterrence” and “with a variety of striking power.” Earlier Monday, it declared seven live-fire zones along the disputed sea border hugging the southern coast of North Korea and warned South Korean fishing boats out of the areas.

The western waters are the most dangerous flash points along the border between North and South Korea. A string of South Korean islands, guarded by marines and heavy artillery, lie just south of the maritime border and within the range of massive arrays of North Korean coastal guns and rocket launchers. The waters were the scene of several naval skirmishes in recent years and an artillery duel in 2010.

The firing zones North Korea had earlier announced lie north of the “northern limit line,” or N.L.L., the sea border South Korea tried to defend, although North Korea does not recognize it.

“We told the North that we will respond powerfully if any of its firing violates the N.L.L.,” the South Korean military had said in a statement earlier Monday.

In 2010, North Korea fired hundreds of artillery rounds into disputed waters, some of them falling south of the N.L.L. Later that year, it shelled one of the South Korean border islands, killing four people and prompting the South to retaliate with its own artillery barrage at North Korean gun positions.

There was no sign of an imminent nuclear test from North Korea, but the South Korean military was operating an emergency response system to promptly handle North Korean provocations, the South Korean Ministry of National Defense said on Monday. The impoverished Communist government has conducted three underground nuclear tests since 2006.