It comes after Pyongyang last week vowed to close roads and railways once seen as symbols of inter-Korean cooperation in an attempt to "completely separate" the two countries.
The Korean People's Army (KPA) described the move as a "self-defense measure to prevent war," saying it was taken in response to military drills in South Korea and the frequent presence of US nuclear forces in the region.
It marks an escalation in hostility at a time when tensions between the Koreas are at their highest levels in years.
After the explosions, the South Korean military said it opened fire on its side of the border in a show of force and stepped up surveillance of the North.