The incursion was a highly unusual incident and it comes during heightened tensions between the rivals over North Korea’s barrage of missile tests.

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” that triggers an automatic operational response from the South.

The South Korean Joint Chiefs of Staff (JCS) said it responded by scrambling F-35 jets and other warplanes. There were no reports of clashes.

The JCS 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 a highly unusual incident and it comes during heightened tensions between the rivals over North Korea’s recent barrage of missile tests.

North Korea’s official KCNA news agency quoted the military as saying it took “strong military countermeasures” after South Korean artillery fire.

‘Tactical nuclear’ drills

Meanwhile, South Korea said North Korea launched yet another ballistic missile towards its eastern waters early Friday but gave no further details.

On Thursday, North Korea said it tested long-range cruise missiles the day previous.

Pyongyang said earlier this week the series of missile launches were “tactical nuclear” drills personally overseen by leader Kim Jong Un, and a response to joint US-South Korea naval exercises.

The tests over the past two weeks were simulated nuclear attacks on key South Korean and US targets, said North Korea, adding 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.