North Korea has severed its military hotline with South Korea, suspending the last direct communication link between the two countries.
The decision coincides with an announcement North Korea's top political leadership will meet in the next few days to discuss an unspecified "important issue" and make a "drastic turn".
The hotline move was relayed by a senior North Korean military official to his South Korean counterpart just before the link was severed.
"Under the situation where a war may break out any moment, there is no need to keep up North-South military communications," the official was quoted as saying by the Korean Central News Agency.
"From now, the North-South military communications will be cut off," he said.
Several weeks ago North Korea severed the Red Cross hotline used for government-to-government communications in the absence of diplomatic relations.
Severing the military hotline could affect operations at the Seoul-funded Kaesong industrial complex in the North, as the hotline was used to organise movement of people and vehicles.
The industrial estate - established in 2004 as a symbol of cross-border cooperation - has remained operational despite the repeated crises in inter-Korean relations.
Cutting the hotline is the latest in a line of threats and moves from Pyongyang that have increased tensions on the Korean peninsula since North Korea's long-range rocket launch in December and its nuclear test last month.
Both events triggered UN sanctions that infuriated Pyongyang, which has spent the past month issuing statements about unleashing an "all-out war".
Earlier, North Korea announced an imminent meeting of the ruling party politburo and launched a scathing attack on South Korea's new president, Park Geun-Hye.
North Korea's Committee for the Peaceful Reunification of Korea, a state body in charge of propaganda and inter-Korean affairs, accused Ms Park of slander and provocation after she made a speech warning the North that failure to abandon its nuclear weapons programme would result in its collapse.
"If she keeps to the road of confrontation... she will meet a miserable ruin," the committee said.
Cho Han-Bum, a Seoul-based analyst at the Korea Institute for National Unification, said North Korea was fast running out of threats and targets for its invective as it sought to bully the international community into negotiating on Pyongyang's terms.
"They are upping the rhetorical ante in every possible way, but the international community is not reacting as it had hoped," he said.
Mr Cho said the coming politburo meeting would probably seek to keep "the momentum going" through some symbolic gesture.
"I envisage a resolution that further raises the alarm, like declaring a top alert for the entire nation beyond the military, or something like that," Mr Cho said.
However, there are concerns in South Korea and beyond that the current situation is so volatile one accidental step could escalate into serious conflict.
On Tuesday, North Korea's military put its "strategic" rocket units on a war footing, with a fresh threat to strike targets on the US mainland, Hawaii and Guam, as well as South Korea.
Pentagon spokesman George Little said US forces were ready to respond to "any contingency", while Japan, which hosts a number of US bases, said its government was "on full alert".
The US and South Korean militaries signed a new pact last week, providing for a joint military response to even low-level provocation by North Korea.