Fighter jets from both countries have appeared in the region.
The South Korean Air Force sent about 30 fighter jets to the inter-Korean border on Thursday after Seoul spotted several North Korean military aircraft in the area.
The incident was reported just hours after North Korea launched two more missiles toward the Sea of Japan in the morning, two days after its last test. The Joint Chiefs of Staff determined that the missiles were launched from the Pyongyang area, adding that they were believed to be ballistic missiles.
UN resolutions prohibit North Korea from testing ballistic missiles of any range, some of which are capable of carrying nuclear warheads.
Pyongyang also fired a medium-range ballistic missile on Tuesday. The last such case occurred five years ago, in 2017.
North Korea has already conducted about 40 missile tests this year.
The operations have led Seoul, Tokyo and Washington to ramp up joint military exercises recently, and on Sept. 30, they conducted their first trilateral anti-submarine warfare maneuvers in five years off the Korean Peninsula.