The US has denounced North Korea for threatening a “sacred war” against the South, whose military has been holding live-fire drills near the border.
The state department’s Philip Crowley told the BBC there was no justification for Pyongyang’s “belligerent words”.
In a day of rising tension, Seoul and Pyongyang traded strong rhetoric, with the South warning of a “powerful response” to any attack from the North.
A month ago, the North fired on a Southern island, killing four people.
Thursday’s speech by Armed Forces Minister Kim Yong-chun marks the strongest statement from Pyongyang since the attack on Yeonpyeong island.
Analysts believe the hard-line stance might be timed to coincide with the 19th anniversary of North Korean leader Kim Jong-il taking control of the armed forces, which will be marked on Friday.
“We’ve heard this language before,” said Mr Crowley in an interview with BBC’s Newshour.
“Unfortunately sometimes that kind of language is followed by irresponsible actions, whether it’s a missile test, a nuclear test or the shelling of South Korea, as occurred last month.”
He added that the North would get no reward for its “provocative actions”.
China, the North’s only major ally, also issued a statement asking both parties on the peninsula to remain calm.
Pyongyang is frequently accused of sabre-rattling in order to strengthen its hand in negotiations with other countries over its nuclear ambitions.
But the North insists that it is the victim, and repeatedly accuses the South of preparing for war by holding military drills on the border.
Kim Yong-chun, quoted by state news agency KCNA, said the North was “getting fully prepared to launch a sacred war of justice”, and also threatened to use a “nuclear deterrent”.
Despite possessing enough plutonium to create a bomb, the North is not thought to have succeeded in building a nuclear weapon.
International talks over Pyongyang’s nuclear ambitions halted in April 2009, when the North walked out and expelled UN nuclear inspectors.
The US has refused to resume the talks until North Korea recommits to its past promises to give up its nuclear-weapons programme.