North Korea's constitution now defines the South as a "hostile state", according to state media, in the first mention of what Pyongyang's recent constitutional revisions entailed.

State newspaper Rodong Sinmun reported the change as an "inevitable and legitimate measure", at a time when tensions between the Koreas are at their highest point in years.

The North on Tuesday blew up roads and railways connecting it to South Korea - a move which state media described as "part of the step-by-step implementation to thoroughly separate [the Koreas]".