Amid tensions over the North’s third nuclear test last month and ensuing United Nations sanctions, North Korea had already shut down Red Cross hotlines with South Korea and a communication line with the American military command in South Korea. But its decision to cut off military hotlines with South Korea on Wednesday was taken more seriously in Seoul because the two Koreas have used those four telephone lines to control daily cross-border traffic of workers and cargo travelling to the North Korean border town of Kaesong.

The two countries run a joint industrial park at Kaesong, the last standing symbol of inter-Korean cooperation that has survived the political tensions of recent years. Seoul officials said 887 South Korean workers were in Kaesong on Wednesday. The traffic was running normal on Wednesday, South Korean officials said, indicating that the North Korean military did not go so far as to stop cross-border economic exchanges.