Referenews

In response, Seoul scrambled jet fighters, attack helicopters and other warplanes after having issued warning messages and fired warning shots. A light-attack aircraft crashed shortly after being deployed, with both pilots having escaped safely, according to South Korea’s semiofficial Yonhap News Agency.

Drones have been crucial to both sides in Russia’s war, though especially to Kyiv, as it allows them to inflict significant damage on Moscow’s far bigger, better-equipped forces. Taiwan has warned it would take countermeasures against Chinese drones deemed a security threat if they fail to leave after warnings. North Korea’s unmanned aerial vehicles are relatively unproven, though are a priority.

Four of the drones vanished from South Korea’s radar hours after being detected; the fifth was tracked returning to the North after spending roughly three hours in South Korea, Seoul’s military said. In recent years, lax border controls have dogged South Korea’s government, with individuals slipping undetected in and out of the North by sea and land.

Now South Korea’s potential vulnerabilities in the skies are likely to be questioned, as tensions have escalated on the Korean Peninsula to their highest levels in years.

The

Kim Jong

Un regime has unleashed a historic number of missile launches this year, including one on Friday, and has threatened to conduct more tests. The Seoul government under President

Yoon Suk

-yeol, a conservative who took office in May, has adopted a tougher stance against Pyongyang and sought to deepen its security alliance with Washington.

The North Korean drone activity adds another layer of unease, as flights were temporarily suspended Monday at South Korea’s two largest airports. The drones’ intrusion represented a clear act of provocation by North Korea, said South Korea’s Joint Chiefs of Staff, promising a thorough and stern response.

Seoul officials said they didn’t know whether the drones had been equipped with weapons.

The drones were detected in airspace near the South Korean city of Gimpo at around 10:25 a.m. on Monday. South Korea’s military, as part of the corresponding measures, said it had also conducted manned and unmanned reconnaissance, including even in areas north of the two countries’ Military Demarcation Line.

Drones fit into a central North Korean military pursuit—updating its unmanned aerial surveillance. In a January 2021 policy speech, Mr. Kim outlined a new five-year weapons strategy that noted “the most important research” being done to develop spy drones and other reconnaissance technology. Pyongyang has touted its recent advances on a military-reconnaissance satellite that should be completed by next spring.

Since at least 2014, South Korean officials have recovered drones believed to be from North Korea—and which had crashed midflight. Pyongyang in previous years denied sending the drones. The last suspected North Korean drone intrusion came in 2017, dispatched on a mission to snap photos of a controversial U.S. missile-defense battery in South Korea.

Pyongyang’s state media didn’t have immediate comment on Monday’s drone activity.

North Korea has ridiculed the new Yoon administration, conducted missile tests that simulated strikes on South Korea and brushed off any offers from Seoul to talk. On a single day in November, Pyongyang launched ballistic missiles from at least eight different locations off its eastern and western coasts and fired nearly two dozen missiles. One flew so close to a South Korea island it triggered an air-raid warning.

In response to the Kim regime’s growing nuclear threat, South Korea has often responded with corresponding missile drills and scrambling of jet fighters. Seoul has also participated in more joint military drills with Washington and Tokyo. Mr. Yoon, who took office in May, has vowed to accelerate plans to establish a “kill chain” system designed to launch pre-emptive strikes against the North.

In November, President Biden, in a meeting with the leaders of Japan and South Korea, reaffirmed the U.S. commitment to strengthening deterrence against North Korea with the full range of capabilities, including nuclear, according to a statement from the three countries. Among the potential moves could be dispatching more frequently to the region American strategic assets, such as nuclear-powered aircraft carriers or submarines.

Days after that trilateral meeting ended in Cambodia, North Korea’s foreign minister,

Choe Son Hui,

in state media, protested America’s “bolstered offer of extended deterrence” and warned such agreements would bring the Korean Peninsula to an unpredictable phase.

Such “bluffing military activities” by the U.S. and its allies would trigger a “fiercer” North Korea military counteraction, Ms. Choe said in the Nov. 17 statement.

Washington and Seoul last week conducted combined air exercises that sought to simulate escorting and protecting U.S. strategic B-52H bombers, for when such assets are deployed to the Korean Peninsula, according to U.S. Forces Korea.

The U.S. and South Korea had said in November they needed to expand and restart bigger military exercises, after a multiyear hiatus as the Trump administration engaged in talks with Pyongyang.

The two countries have some 20 joint training programs planned for the first six months of 2023, Seoul’s defense ministry said last week.

The Kim regime sees those military exercises as little more than dress rehearsals for an invasion against North Korea.

Under a September 2018 accord, the two Koreas had agreed to cease hostile activities against each other, including military drills near the border. But the Yoon administration has faced pressure from ruling party lawmakers in Seoul to terminate the inter-Korean military agreement, especially if North Korea conducts another nuclear test as U.S. and South Korean officials have warned could occur.

Write to Timothy W. Martin at Timothy.Martin@wsj.com