North Korea now defines South Korea in its constitution as a fully hostile country, so it has cut off the highway and railway service, North Korean state news agency KCNA reported.
The agency said that this was an inevitable and legitimate measure taken in accordance with the North Korean laws and due to the serious security circumstances running to the unpredictable brink of war owing to the grave political and military provocations from Seoul.
The report also read, citing a defense ministry spokesman, that North Korea would continue to take measures to permanently fortify the closed southern border.
Later on Thursday, Seoul strongly condemned that Pyongyang had made constitutional amendments, describing South Korea as a hostile state, and that it "ignored the aspirations" of the two peoples to unite.
"South Korean government strongly condemns these actions, regarded as anti-unification and anti-humanitarian, which ignore the aspirations of both the South and North Korean people for unity," the South Korean Unification Ministry said in a statement.