North Korea Brackets the South With a Barrage of Missiles

In response, South Korea fired three air-to-ground missiles from warplanes into the sea north of the disputed border. North Korea shelled a different South Korean island in 2010 and a South Korean military unit near the Demilitarized Zone in 2015, but Wednesday was the first instance of Pyongyang flying a missile south of the countries’ disputed maritime border. South Korean authorities are analyzing whether the missile that crossed the border had gone off course or whether the flight path was intentional.

Following the barrage of missiles, North Korea fired about 100 artillery shells in the buffer zone around the disputed maritime border. South Korea’s military sent messages warning North Korea to stop firing in violation of a 2018 inter-Korean military agreement.

South Korean President Yoon Suk -yeol called an emergency meeting shortly after the launches, vowing a swift and firm response so that North Korea pays a price for its provocation. Japanese Prime Minister Fumio Kishida called the repeated missile tests “absolutely unacceptable.”

South Korea’s National Security Council also protested the launch, which came during a national mourning period over the deaths of at least 156 people who were crushed in a crowded nightlife district in Seoul. The provocation reflected a lack of regard for the recent loss of life in South Korea, the NSC said in a statement.

North Korea is protesting the recent joint military drills between Washington and Seoul and escalating tensions to create pretext for a larger provocation, potentially its next nuclear test, said Yang Moo-jin, a professor at the University of North Korean Studies in Seoul. “Once South Korea takes retaliatory action to its provocations, North Korea can use that as justification to showcase its nuclear capacity,” Mr. Yang said.

In recent weeks, South Korean officials had warned that North Korea could spring a surprise, localized attack and said they were making preparations to respond.

Seoul’s military first detected four short-range ballistic missiles launched off the North’s west coast around 6:51 a.m. local time. Then around 8:51 a.m., North Korea fired three more short-range ballistic missiles from near the North’s eastern coastal city of Wonsan. One of the three missiles crossed over the disputed maritime border, which Seoul’s military said was “unprecedented and unacceptable.” At 9:12 a.m., around 10 more missiles, including ballistic and surface-to-air missiles, were fired off the North’s east and west coasts.

The air-raid warning was issued around 8:55 a.m. at Ulleung Island, and news programs showed emergency alerts that read: “Evacuate to the nearest underground shelter and listen to broadcast.” The air-raid warnings were lifted at 2 p.m. local time.

The spree of launches came after North Korea demanded the U.S. and South Korea stop joint military drills, calling them a “ceaseless and reckless” military provocation. In recent months, Washington and Seoul have conducted a series of military exercises, including air drills that started Monday involving thousands of service members and hundreds of warplanes that staged mock attacks.

Pyongyang has blamed Washington and Seoul for escalating tensions on the Korean Peninsula by conducting military drills and warned of “more powerful follow-up measures,” according to a Tuesday state-media report. North Korea has called its recent missile tests and artillery firing a response to the drills, denouncing the joint military exercises as a rehearsal for invasion.

On Tuesday, a State Department spokesman said the joint military exercises were purely defensive in nature and reiterated that the U.S. harbors no hostile intent toward North Korea.

The following day, a senior North Korean official warned that the U.S. and South Korea will pay “the most horrible price in history” if they attempt to use armed forces against North Korea, according to a state-media report.

North Korea often issues angry statements blaming the U.S. and South Korea for hostile activities to internally gain support for continued provocations while also publicizing its justification to the international community, Pyongyang watchers say.

“It makes it easier for China and Russia to have North Korea’s back and blame the U.S. when North Korea does conduct its next nuclear test,” said Kim Yong-hyun, a professor of North Korean studies at Dongguk University in Seoul.

Before Wednesday, Pyongyang’s biggest single-day missile barrage was on June 5, when Pyongyang fired eight short-range missiles from four locations. Weapons experts say such tests are aimed at demonstrating the Kim regime’s ability to quickly deploy and launch missiles from various sites at once, making it difficult for the U.S. and South Korea to intercept.

North Korea refrained from conducting a missile test during China’s Communist Party congress in October, but fired artillery shells into the buffer zone along the maritime border several times. Pyongyang has conducted more than two dozen rounds of missile launches this year, the most it has ever done in a single year.

Washington and Seoul officials have said North Korea has completed preparations to conduct its seventh nuclear test, the first since 2017. Officials from Washington, Tokyo and Seoul warned of an unparalleled scale of response if Pyongyang conducts a nuclear test. The U.S. Department of Defense said a nuclear attack by North Korea would result in the “end of that regime,” in its National Defense Strategy report published last Thursday.

North Korea’s ballistic-missile launches violate United Nations Security Council resolutions and typically draw recriminations from the U.S., Japan and others. U.S.-led efforts to impose additional sanctions on North Korea have been blocked by Russia and China.

—Chieko Tsuneoka in Tokyo contributed to this article.

Write to Dasl Yoon at dasl.yoon@wsj.com