Looking for His glorious appearing! Titus 2:13

The warning came as South Korean President Park Geun-hye reiterated on Monday her commitment to engage with the North, despite what she called "the dual nature" of ties, saying the "door was always open to dialogue".

South Korea said the North Korean firing was a "provocative act" that had violated the truce suspending their 1950-53 war and the complaint was reiterated in a notice the South sent the North late on Sunday.

"It is a stern warning of a strong response in the event of further provocation," South Korean Defence Ministry spokesman Kim Min-seok told a briefing.

The exchange of fire on Friday began after North Korea fired shots at balloons carrying leaflets sent towards the North by South Korean activists critical of the North's leadership.

North Korea has long criticised the leaflet drops as provocative and it has threatened to respond to them with force. But before Friday, it had never done so.

No one was hurt in the firing.

North Korea's state media said on Saturday that expected talks with the South to try to improve ties were in danger of being cancelled because authorities in South Korea had allowed the activists to float their balloons.

"The leaflet-scattering operation ... was a premeditated and deliberate politically motivated provocation perpetrated under the backstage wire-pulling of the U.S. and the South Korean authorities," the North's KCNA news agency said.