South Korea and the United States raised the military alert level for the peninsula on Thursday.
It came after the communist North warned the truce ending the Korean War was dead and it is ready to attack.
North Korea ramped up tensions this week with a series of provocations rarely seen since the 1950-53 Korean War, including war threats, missile launches and a nuclear test that puts it closer to having an atomic bomb.
The joint command for the 28,500 US troops that support South Korea's 670,000 soldiers has raised its alert a notch to signify a serious threat from North Korea.
It is the highest threat level since the North's only other nuclear test in October 2006.
UN sanctions likely
North Korea looks certain to face fresh sanctions for defying a United Nations resolution by exploding a nuclear device for a second time.
Key powers on the Security Council have agreed in principle that the country must face sanctions.
Western diplomats say the agreement came out of talks on Tuesday among the five permanent members - the United States, Britain, France, China and Russia - and Japan.
South Korea, which is not a council member, was also present.
The Security Council unanimously condemned the test on Monday, saying it was a violation of international law.
Diplomats close to the latest talks predict the seven countries involved will not have a draft resolution ready to circulate to the full 15-nation council until next week at the earliest.
They say the council will not seek new punitive measures but will look for ways to enforce and expand sanctions agreed to after North Korea's first nuclear test.