The incursion is a highly unusual incident and occurs at a time of heightened tensions after a barrage of missile tests by Pyongyang.
South Korea has scrambled fighter jets after North Korea flew warplanes close to its border before launching another ballistic missile.
About a dozen North Korean aircraft came as close as 12km (7 miles) north of the inter-Korean border between late Thursday and early Friday, South Korea’s Joint Chiefs of Staff said in a statement.
They crossed a Seoul-set “reconnaissance line”, which triggers an automatic operational response from the South.
South Korea’s military said it responded by scrambling F-35 jets and other warplanes. There were no reports of weapons fired.
The Joint Chiefs of Staff said the South Korean air force “conducted an emergency sortie with its superior air force, including the F-35A, and maintained a response posture while carrying out a proportional response manoeuvre corresponding to the flight of a North Korean military aircraft”.
The incursion was highly unusual and happened as tensions have been high between the two neighbours over North Korea’s recent barrage of missile tests.
On Friday, the North’s military said its latest actions came in response to a “provocative” South Korean artillery exercise near the border.
North Korea’s official KCNA news agency quoted its military as saying it took “strong military countermeasures” after South Korean artillery fire.
The Korean People’s Army “sends a stern warning to the South Korean military inciting military tension in the frontline area with reckless action”, said a statement.
‘Tactical nuclear’ drills
Meanwhile, South Korea said North Korea launched yet another ballistic missile into the sea off its east coast early on Friday, but it gave no further details.
On Thursday, North Korea said it had tested long-range cruise missiles a day earlier.
Pyongyang said the series of missile launches was “tactical nuclear” drills personally overseen by leader Kim Jong Un and a response to joint US-South Korean naval exercises.
The tests over the past two weeks were simulated nuclear attacks on key South Korean and US targets, North Korea said, adding that they were meant as a warning to Seoul and Washington over their manoeuvres.
The launches, part of its record-breaking run of weapons tests this year, are seen as an attempt by Kim to acquire a more intimidating arsenal to pressure its rivals to accept his country as a legitimate nuclear state and lift economic sanctions.