North Korea has said that it has cut off a key military hotline with South Korea that allows cross-border travel to a jointly run industrial complex in the North.

The move jeopardises the last major symbol of inter-Korean cooperation.

North Korea recently cut a Red Cross hotline with South Korea and another with the US-led UN command at the border between the Koreas.

However, there is still a hotline linking aviation authorities in the two countries.

North Korea's chief delegate to inter-Korean military made the announcement in a statement sent to his South Korean counterpart.

"The North-South military communications will be cut off and the members of the North side at the military communications liaison office in the zone under the control of the North and the South in the west coastal area will stop their activities from this moment," he said.

The hotline is important because the Koreas use it to communicate as hundreds of workers travel back and forth to the Kaesong industrial complex.

South Korean officials say more than 900 South Korean workers were in Kaesong today.

There has been no word about how cutting the communications link would affect their travel back to South Korea.

North Korea, angry over routine US-South Korean drills and recent UN sanctions punishing it for its 12 February nuclear test, has made several threats recently, including vows to launch a nuclear strike against the US.

Kaesong is operated in North Korea with South Korean money and know-how and a mostly North Korean workforce.

It provides a badly needed flow of hard currency to a country where many face food shortages.