North Korea says it is scrapping all non-aggression pacts with South Korea, closing its hotline with Seoul and closing the crossing point between the two countries.
The announcement follows a fresh round of UN sanctions against Pyongyang over its controversial nuclear programme.
The UN resolution was in response to the North's nuclear test last month.
Earlier, Pyongyang vowed to use its right to a pre-emptive nuclear strike against its "aggressors".
The announcement, carried on North Korea's state news agency, says the North is cancelling all non-aggression pacts with the South and closing the main Panmunjom border crossing inside the Demilitarized Zone.
It said it was notifying the South that it was "immediately" cutting off the North-South hotline.
North Korean leader Kim Jong-un is also reported to have visited front-line military units that were involved in the 2010 shelling of a South Korean island.
He is said to have urged the soldiers there to keep themselves ready to "annihilate the enemy" at any time, and reconfirmed so-called "enemy targets" on five islands in the West Sea.
Earlier, the UN Security Council unanimously backed Resolution 2094, imposing the fourth set of sanctions against the North.
The resolution targets North Korean diplomats, cash transfers and access to luxury goods.
It imposes asset freezes and travel bans on three individuals and two firms linked to North Korea's military.
Speaking after the vote, US ambassador to the UN Susan Rice said the document "strongly condemns" Pyongyang's actions.
She said the sanctions would "further constrain" North Korea's ability to develop its nuclear programme.
Ms Rice also warned that the UN would "take further significant actions" if Pyongyang were to carry out another nuclear test.
--Courtesy of BBC News