North Korea has cut its last military hotline with Seoul saying there was no need to for communications between the countries because "a war may break out at any moment".
The closure is the latest of many threats and provocations from the North which is angry over US-South Korean military drills and recent UN sanctions over its nuclear test.
The link had been essential in operating the last major symbol of inter-Korean cooperation: an industrial complex in the North that employs hundreds of workers from the South. It was used by the two countries' militaries to arrange for workers and shipments to cross their heavily armed border.
There was no word about the impact on South Korean workers who were at the Kaesong industrial complex. When the link was last cut in 2009, many South Koreans were stranded in the North.
Outside North Korea, Pyongyang's actions are seen in part as an effort to spur dormant diplomatic talks to wrest outside aid, and to strengthen internal loyalty to young leader Kim Jong Un and build up his military credentials.
North Korea's action was announced in a message that its chief delegate to inter-Korean military talks sent to his South Korean counterpart. "Under the situation where a war may break out any moment, there is no need to keep North-South military communications," he said. "North-South military communications will be cut off."
Seoul's Unification Ministry, which is in charge of relations with the North, called the move an "unhelpful measure for the safe operation of the Kaesong complex." It said only three phone hotlines remain between the North and South, and they are used only for exchanging information about air traffic.
About 750 South Koreans were in Kaesong and the two Koreas had normal communications earlier in the day over the hotline when South Korean workers travelled back and forth to the factory park as scheduled. Kaesong is operated in North Korea with South Korean money and know-how and a mostly North Korean work force. It provides badly needed hard currency in North Korea, where many face food shortages.
Other examples of joint inter-Korean cooperation have come and gone. The recently ended five-year tenure of hard-line South Korean President Lee Myung-bak saw North-South relations plunge.
North Korea last cut the Kaesong line in 2009, in protest at South Korean-US military drills. North Korea refused several times to let South Korean workers commute to and from their jobs, leaving hundreds stranded in North Korea. The country restored the hotline and reopened the border crossing more than a week later, after the drills were over.