North Korea has vowed to scrap all peace pacts with the South after the United Nations imposed tough new sanctions on the country.
The warning came as state media released pictures of Kim Jong-Un meeting troops on islands believed to be in the east of the country.
The news agency KCNA said he talked of "all-out war" during the visit - and quoted him as telling troops to "make the first gunfire" in response to any attack.
He said the slightest provocation would result in his immediate order for a "great advance" along the frontline, the agency reported.
The UN sanctions, which aim to curtail the country's nuclear and missile programme, include financial restraints and a crackdown on attempts to ship and receive banned cargo.
The restrictions prompted North Korea to threaten the US with pre-emptive nuclear strikes.
China, North Korea's only major ally, said it wanted the sanctions to be implemented in full. It agreed to the restrictions after three weeks of negotiations.
A statement released by China's Foreign Ministry called the sanctions a "necessary and moderate response" to an underground nuclear test last month .
The test, just north of a site where Pyongyang carried out experiments in 2006 and 2009, was described by the UN Security Council as a "clear threat" to world peace.
Susan Rice, the US ambassador to the UN, added: "When North Korea tries to move money to pay for its nuclear and ballistic missile programmes, countries must now block those transfers, even if the money is being carried in suitcases full of cash."