By Choe Sang Hun

SEOUL — North Korea fired nearly two dozen missiles Wednesday (2) in its largest deployment on a single day, an escalation in recent tensions that triggered an air-raid alert on a populated island in the South and provoked a military warning from Seoul.

North Korea launched at least 23 missiles off its east and west coasts, one of which landed 103 miles northwest of the outlying island of Ulleung, where people took cover. South Korea, in response, fired missiles from fighter jets into waters near the North’s territory.

The exchange between the two marked the first time that many missiles had been launched across the Koreas’ maritime border, the South’s Defense Ministry said, though all of them fell into international waters.

Lt. Gen. Kang Shin Chul, the South Korean military’s chief director of operations, called the launch “a highly unusual and intolerable act.”

“The siren started blaring at 8:55 a.m. and we got a message from our government computer system saying that this is ‘a real-life situation,’” not a mock drill, said Chung Young-hwan, a local official on Ulleung. “We took refuge at an underground shelter for three or four minutes before coming out again.”

Chung said that the siren was triggered across the island but residents were not immediately told the reason for it. “We knew that it was a North Korean missile when we saw news flashes,” he said.

The South’s military later said that its F-15K and KF-16 fighter jets had fired three precision-strike air-to-land missiles into international waters not far from the North’s own territorial waters as a warning. South Korea’s transportation ministry barred air traffic from the area around Ulleung until Thursday (3) morning.

The exchange added to the tensions that have been building on the Korean Peninsula in recent months. The South’s president, Yoon Suk Yeol, said the launch near Ulleung was “tantamount to violating South Korea’s territorial waters,” according to his office.

North Korea has conducted 28 weapons tests this year that involved ballistic and other missiles — more than in any previous year — in defiance of United Nations Security Council resolutions that ban the country from testing ballistic missiles as well as nuclear devices.

South Korean defence officials said that they were still studying data to determine the types of missiles North Korea had fired but that at least seven were ballistic missiles. The North also fired about 100 artillery rounds and rockets into a “buffer zone” just north of the eastern maritime border, officials said.

The launch Wednesday was the North’s most daring missile test since Oct. 4, when it fired an intermediate-range ballistic missile that flew over northern Japan. That missile flew farther than any other missile the North has tested.

Though the South said that sending missiles across the maritime border was a new development, the Koreas have exchanged artillery and machine-gun fire across their border in the past. The North has fired hundreds of rockets and artillery shells into waters not far from the maritime border in recent weeks.

In 2010, the North shelled Yeonpyeong, a South Korean border island off the country’s west coast, killing four people.

The missile launches Wednesday came as South Korea was mourning the deaths of more than 150 people in a crowd crush in Seoul during Halloween festivities last weekend. Yoon’s office said the timing “clearly exposed the true face of the North Korean regime and its flouting of humanity and humanitarianism.”

They also came two days after South Korea and the United States began an annual joint military exercise, which this year involved 240 aircraft and thousands of military personnel from both countries. The planes were expected to perform about 1,600 sorties, the most ever for the annual drill, which the allies say is meant to ensure preparedness in light of the growing threat from North Korea.

North Korea has called such drills rehearsals for invasion. On Monday, when the exercises began, the North protested that the allies’ aircraft were practicing “striking the strategic targets of the DPRK,” referring to the Democratic People’s Republic of Korea, the North’s official name.

“If the US continuously persists in the grave military provocations, the DPRK will take into account more powerful follow-up measures,” the North’s Foreign Ministry said at the time.

North Korea’s leader, Kim Jong-un, has repeatedly vowed to expand the country’s nuclear and missile forces. He is believed to see them as essential to ensuring his regime’s security, boosting his leverage in any arms control talks with Washington and tipping the peninsula’s balance of military power in the North’s favour.​

In recent years, Kim has increasingly turned his attention to testing various shorter-range missiles, which the North said were harder to intercept and could deliver nuclear warheads to the South and to Japan, another US ally in the region.

The sense of self-empowerment that a nuclear arsenal provides could make the North increasingly daring in its military provocations, analysts said.

The air-raid alert on Ulleung ​revived memories of the North’s deadly shelling of Yeonpyeong in 2010, though that island lies much closer to the North, just miles from the western maritime border that South Koreans call the Northern Limit Line, which the North does not recognize. But unlike in 2010, analysts noted, the North now openly threatens to use nuclear weapons against the​ South.

“The North Korean test today was predictable in that it happened during a joint South Korea-US military ​drill, but shocking ​in that the missile flew toward Ulleung island,” said Lee Byong-chul, a North Korea expert at the Institute for Far Eastern Studies at Kyungnam University in Seoul. ​“North Korea is not backing down from its power-for-power confrontation and brinkmanship with the US and South Korea. We will likely see more tensions.”

-New York Times