South Korean President Park Geun-hye has vowed "strong retaliation" to any provocation by North Korea, after Pyongyang declared it was formally at war with Seoul.
Meeting senior military officials and Defence Minister Kim Kwan-jin on Monday, Park said: "If there is any provocation against South Korea and its people, there should be a strong response in initial combat without any political considerations."
Meanwhile, the White House played down the North's threat, saying that despite bellicose rhetoric, the US administration has not seen changes in Pyongyang's military posture.
Park, who focused her election campaign last year on a promise not to be blackmailed by the North.said she took the near-daily stream of threats emanating from the North "very seriously".
"I believe that we should make a strong and immediate retaliation without any other political considerations if [the North] stages any provocation against our people," she said.
Military tensions between the two neighbours have been running high for weeks, with the North stepping up its hostile rhetoric against Seoul and Washington.
In response, US officials said on Monday that a Japan-based US Navy guided-missile destroyer capable of shooting down ballistic missiles had been positioned slightly closer to the Korean peninsula, though still within its usual operating area.
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White House spokesman Jay Carney said: "I would note that despite the harsh rhetoric we are hearing from Pyongyang, we are not seeing changes to the North Korean military posture, such as large-scale mobilisations and positioning of forces."
In protest at joint South Korean-US military drills, North Korea last month declared it was ripping up the armistice that ended the 1950-53 Korean War void and threatened a "pre-emptive nuclear strike" on South Korean and US targets.
On Saturday, it announced that it had formally entered into a "state of war" with South Korea.
Seoul and Washington have warned of severe repercussions in the event of any aggression, with the US deploying nuclear capable B-52 and B-2 stealth bombers, as well as F-22 stealth fighters over South Korea as a "deterrence".