(ATTN: UPDATES with more details in para 9)
SEOUL/HOENGSEONG, Dec. 26 (Yonhap) -- Five North Korean drones intruded across the inter-Korean border with one of them having flown over northern Seoul on Monday, officials here said, prompting South Korea to send its own unmanned vehicles into the North in a "corresponding" step.
The South's military detected the unmanned aerial vehicles (UAVs) in border areas of Gyeonggi Province from 10:25 a.m., according to the Joint Chiefs of Staff (JCS). The UAVs were 2 meters wide or smaller.
The vehicles flew across the Military Demarcation Line separating the two Koreas, and were spotted flying in those areas in Gimpo, Ganghwa Island and Paju, leading to temporary suspensions of civilian flights. It marks the first such intrusion in five years.
One of them flew all the way to the northern part of Seoul, while the others hovered largely in and around Ganghwa Island, west of the capital, in what officials here view as a move to cause "disruption" to the South.
The South issued warning messages, fired warning shots, and scrambled fighters, attack helicopters and other warplanes to remove them, while it remained unconfirmed whether the vehicles carried any weapons, the JCS said.
At some point, military choppers are said to have fired some 100 shots at the North's vehicles in a western coastal area but failed to shoot them down.
The JCS noted it limited its operations to avoid any damage to civilians.
A South Korean pilot identified one of the drones by sight. It appeared akin to a North Korean drone that crashed in the South's northeastern county of Inje in 2017, according to Seoul officials.
The operation against the five drones lasted around five hours in total. One of them returned to the North after around three hours of flight in the South, while the others vanished from the South's radar after their detection here in the afternoon, the JCS said.
In a corresponding step, the South sent its own manned and unmanned assets to the border areas, with some into the North to carry out surveillance and other operations, including taking photos of "key enemy military facilities," according to the JCS. All of them returned home safely.
"This is a clear act of provocation by North Korea that encroached upon our territorial air," Maj. Gen. Lee Seung-o, director of operations at the JCS, told a press briefing. "Our military will respond thoroughly and sternly to such a North Korean provocation going forward."
During its operations in the South, the military also deployed a KA-1 light attack aircraft, but for an unknown reason, it crashed in Hoengseong County, about 140 kilometers east of Seoul, at 11:39 a.m. Both of the pilots escaped safely.
Following the drone infiltrations, National Security Adviser Kim Sung-han led a "real-time" response, an aide to President Yoon Suk Yeol told reporters on condition of anonymity. The National Security Council did not convene.
The two Koreas' deployment of drones into each other's territory appeared to have breached both the 2018 inter-Korean military accord on reducing tensions and the armistice that ended the 1950-53 Korean War, observers noted.
The North's drone operations have been a source of growing security concerns here, as they could be used for spy operations, as well as potential attack missions against the South.
The South announced its discovery of North Korean drone operations south of the border in 2014 and 2017, sounding an alarm against security threats posed by those unmanned vehicles.
The reclusive state is estimated to have as many as 1,000 drones in an apparent effort to offset the airpower advantage of the South boasting advanced combat aircraft, as well as to bolster its reconnaissance capabilities, analysts have said.
Among them is the "Banghyun" series thought to have been made based on an imported Chinese D-4 drone system. They also include the "durumi" multi-purpose vehicle thought to be capable of both surveillance and attack missions.
Concerns have persisted that some North Korean drones could carry chemical and biological weapons for use in potential terrorists operations.
In 2017, the North caught the South off guard by sending across the border a drone equipped with a camera, which took photos of a southern U.S. military base operating a THAAD missile defense system.
The latest violation of the South's territorial air raised tensions anew after the North fired two short-range ballistic missiles Friday and two medium-range ballistic missiles five days earlier.