Closed-circuit TV footage of North Korean military exploding parts of inter-Korean roads on Tuesday. (South Korean Joint Chiefs of Staff)

North Korea is building unidentified structures on an eastern inter-Korean road that it blew up last week, a source said Thursday, amid heightened cross-border tensions over the North's border activities.

Last Tuesday, North Korea blew up sections of the eastern Donghae Line and the western Gyeongui Line, just north of the border, days after vowing to cut off all roads and railways linked to the South and build front-line defense structures.

Since the explosions, South Korea's border surveillance assets have spotted the North leveling the ground on the two disconnected roads and recently installing the structures on the Donghae road, according to the military source.

The structures are suspected to possibly serve as the frame for concrete border barriers.

The source said the military has yet to detect signs of the North building such structures on the Gyeongui road.

This month's explosions marked the complete dismantling of inter-Korean land routes once seen as symbols of reconciliation and cooperation between the two Koreas.

North Korea has been wiping out traces of unification after its leader Kim Jong-un late last year defined the Koreas as "two hostile states" and said there is no point in seeking reconciliation and unification with South Korea. (Yonhap)