North Korea says it is scrapping all non-aggression pacts with South Korea, closing its hotline with Seoul and shutting their shared border point.
The announcement follows a fresh round of UN sanctions punishing Pyongyang for its nuclear test last month.
Earlier, Pyongyang said it reserved the right to a pre-emptive nuclear strike against its "aggressors".
The US said it took the threats seriously, but that "extreme rhetoric" was not unusual for Pyongyang.
The North Korean announcement, carried on the KCNA state news agency, said the North was cancelling all non-aggression pacts with the South and closing the main Panmunjom border crossing inside the Demilitarized Zone.
It also said it was notifying the South that it was "immediately" cutting off the North-South hotline.
The hotline, installed in 1971, is intended as a means of direct communication at a time of high tension, but is also used to co-ordinate the passage of people and goods through the heavily fortified Demilitarized Zone.
KCNA said the hotline, which has been severed several times before, "can no longer perform its mission due to the prevailing grave situation".
North Korean leader Kim Jong-un is also reported to have visited front-line military units that were involved in the shelling of a South Korean island in 2010.
KCNA said he had 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.
The US, the main focus of North Korean ire, said it was capable of protecting itself and its allies from any attacks.
"One has to take what any government says seriously," State Department spokeswoman Victoria Nuland said of the nuclear threat.
"It is for that reason that I repeat here that we are fully capable of defending the United States. But I would also say that this kind of extreme rhetoric has not been unusual for this regime, unfortunately."
Senator Robert Menendez, who chairs the Senate Foreign Relations Committee, said North Korea knew what the cost of any attack would be.
"I don't think that the regime in Pyongyang wants to commit suicide, but that as they must surely know, that would be the result of any attack on the United States," he said.
On Thursday, the UN Security Council in New York 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.
South Korea's ambassador to the UN, Kim Sook, said it was time for North Korea to "wake up from its delusion" of becoming a nuclear state.
"It can either take the right path toward a bright future and prosperity, or it can take a bad road toward further and deeper isolation and eventual self-destruction," he said.
US ambassador to the UN Susan Rice said the sanctions would "further constrain" North Korea's ability to develop its nuclear programme.
She warned that the UN would "take further significant actions" if Pyongyang were to carry out another nuclear test.
Chinese Foreign Ministry spokesman Qin Gang issued a statement supporting the UN resolution and describing it as a "moderate response".
He said China - North Korea's sole ally - urged "relevant parties" to stay calm and said the main priority was to "defuse the tensions, bring down heat" and restart negotiations with Pyongyang.
Before the UN vote, Pyongyang accused the US of pushing to start a war.
"As long as the United States is willing to spark nuclear war, our forces will exercise their right to a pre-emptive nuclear strike," said North Korea's foreign ministry, in a statement carried by the KCNA news agency.