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 had a right to carry out a pre-emptive nuclear strike and was pulling out of the armistice that ended the Korean War.
The US said "extreme rhetoric" was not unusual for Pyongyang.
China, which is the North's only major ally, called for both North and South to show restraint and to continue talking.
Beijing rarely criticises its ally, but has criticised the North's nuclear tests and has given support to the UN's sanctions.
It also said it was notifying the South that it was "immediately" cutting off the North-South hotline, saying there was "nothing to talk to the puppet group of traitors about".
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.