North Korea has announced that it will close the land border and cut non-military phone links with South Korea.
North Korea's army has told the South "to strictly restrict and cut off all the overland passages" across the fortified border from 1 December.
A Red Cross office in the North with the only civilian phone link will shut.
If implemented the move will be a blow to inter-Korean relations which have deteriorated since South Korean President Lee Myung-bak came to office.
And it comes despite some progress in international negotiations over dismantling North Korea's nuclear programme.
The border closure decision had been taken because "reckless confrontation" from South Korea was "beyond the danger level", according to the North's official KCNA news agency.
The agency report added: "The South Korean puppet authorities should never forget that the present inter-Korean relations are at the crucial crossroads of existence and total severance."
The phone lines from the Red Cross office in the border village of Panmunjom were the only non-military link between the countries.
Phone and mail links between ordinary people have been cut since the end of the 1950-53 war.
Announcing the closure of the government-controlled Red Cross office, the North criticised the South for co-sponsoring a United Nations resolution criticising its human rights record.
"It is really appalling that the puppet regime is taking a leading role in the racket of a so-called UN resolution on human rights in the North," the statement carried by KCNA said.
A vote on the resolution is expected later in November. It comes a month after a UN human-rights investigator strongly criticised North Korea, urging it to end public executions and provide food for the people not just the elite.
South Korea's unification ministry's spokesman Kim Ho-Nyoun said he did not believe the North intended a complete border closure.
"If the North carries them out, it would have a negative impact on what has been achieved in inter-Korean relations," he said.
South Korea has funded the Kaesong industrial complex just over the border in the North, and a ban on border crossings would make it very difficult for the plant to continue operating.
Some 30,000 North Korean workers are employed by South Korean companies at the complex, and jobs there are highly prized.
Relations between the Koreas have become increasingly strained since February when conservative President Lee Myung-bak took office in Seoul, pledging to get tough with Pyongyang over its nuclear weapons programme.
Last month North Korea threatened to reduce the South to rubble unless it stopped activists sending anti-Pyongyang leaflets attached to balloons into the communist state.
Despite the hostile turn in inter-Korean relations, North Korea has continued to make progress in six-nation talks over its nuclear programme despite frequent setbacks.
It says it is disabling its main nuclear plant at Yongbyon after the US removed the North from a blacklist of state sponsors of terrorism.
The latest escalation in tension comes amid speculation that North Korea's leader Kim Jong-il may have suffered from a serious stroke, though the North has insisted he is in good health and still firmly in charge.