SEOUL, South Korea -- North Korea and South Korea fired hundreds of artillery shells across their disputed western sea border Monday, escalating military tensions a day after the North threatened to conduct more nuclear tests.
South Korean officials said the shells from both sides fell harmlessly into waters from which naval and fishing boats had stayed clear. But the exchange of fire marked the most serious episode along the western sea border since an artillery duel there in 2010.
Earlier Monday, North Korea had told the South it would conduct live-fire military drills in seven zones along the maritime border, which hugs the southern coast of North Korea. Then its artillery pieces and multiple-rocket launchers rolled out of shoreline tunnels and fired 500 shells and projectiles between 12:15 p.m. and 3:30 p.m.
About 100 of them flew across the disputed sea border and fell into South Korean-controlled waters near Baengnyeong Island, said Kim Min-seok, spokesman for the South Korean Ministry of National Defense. Baengnyeong, a South Korean marine garrison, lies only 10 miles from the southwestern tip of North Korea.
In retaliation, South Korean marines fired K-9 self-propelled artillery pieces, pounding North Korean waters north of the disputed sea border with 300 shells, Mr. Kim said.
With guns from both sides rumbling, residents of the five South Korean border islands, including hundreds of children, hurried into bomb shelters. South Korea suspended ferry services to the islands and ordered fishing boats operating near the border waters to return to port.
"This is a premeditated provocation to test our will to defend the maritime border, and if the North provokes again using our response today as an excuse, we will act decisively," Mr. Kim said. "We have increased our vigilance along the western frontier islands and boosted weapons' readiness there."
Artillery exchanges in the disputed waters are not unprecedented, but rising military tensions there indicated that after months of relative calm, hostilities between the two Koreas have begun ratcheting up again. They raised fears that the often-repeated cycle of peace overtures followed by military provocations was resuming on the Korean Peninsula.
"Pyongyang prefers to strike when it sees Washington as weak or distracted, beset by bigger problems," said Lee Sung-yoon, a North Korea expert at Tufts University's Fletcher School of Law and Diplomacy, referring to the North's capital. President Barack Obama "is seen as wavering on Russia and Syria," Mr. Lee said.
Citing as a justification the joint military exercises that Washington and Seoul started in late February, North Korea in recent weeks has test-fired a series of rockets and short- and mid-range ballistic missiles. The tests prompted the U.N. Security Council to warn last week of new action against the country, which is already under heavy sanctions.
On Sunday, Pyongyang threatened "a new form of nuclear test" and warned its military 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."
The two parties in the Korean War never agreed on a western sea border when the three-year conflict ended in a cease-fire in 1953. South Korea tries to defend the so-called northern limit line, which was unilaterally declared by the United Nations. North Korea does not recognize it, claiming another demarcation line farther south.
In 2010, North Korea fired hundreds of artillery rounds into disputed waters, some falling south of the northern limit line. Later that year, it shelled one of the South Korean border islands, killing four people and prompting the South to retaliate with its own barrage against North Korean gun positions.