Annual military drills by South Korea and the US have begun despite North Korean threats to launch a nuclear attack on America.
After the start of the exercises, South Korean officials said their northern counterparts did not answer two calls on a hotline between the sides, apparently following through on an earlier vow to cut the communication channel because of the drills.
Pyongyang has launched a propaganda campaign against the war games, which involve 10,000 South Korean and about 3,000 American troops, and last week's UN vote to impose new sanctions over the North's February 12 nuclear test.
Analysts believe much of that campaign is meant to shore up loyalty among citizens and the military for North Korea's young leader, Kim Jong Un.
They do not think Pyongyang is able to build a warhead small enough to mount on a long-range missile, and the North's military has repeatedly vowed in the past to scrap the 1953 armistice.
North Korea wants a formal peace treaty, security guarantees and other concessions, as well as the removal of 28,500 US troops stationed in South Korea.
North Korea regularly claims South Korea-US drills are a preparation for invasion, but Pyongyang has signalled more worry about the drills that have just begun.
The exercises follow UN sanctions that the North says are the result of US hostility aimed at toppling its political system. North Korea has also warned South Korea of a nuclear war on the divided peninsula and said it was cancelling nonaggression pacts.
Under new President Park Geun-hye, South Korea's Defence Ministry has looked to send a message of strength in response to the latest threats. The ministry warned on Friday that the North's government would "evaporate from the face of the Earth" if it ever used a nuclear weapon.
The White House also said the US is fully capable of defending itself against a North Korean ballistic attack.