Leon Panetta and his counterpart Kim Kwan-Jin also described the North’s uranium enrichment programme (UEP) revealed last November as a “grave threat” which gave the communist state a second path to develop atomic weapons.
The ministers made their commitments on Friday ,in a joint statement at the end of security talks.
Inter-Korean ties have been tense since Seoul accused its neighbour of torpedoing a warship in March 2010 near the Yellow Sea border with the loss of 46 lives.
The North denied responsibility for the sinking but shelled a border island last November, killing four South Koreans including civilians.
“Pyongyang has demonstrated its readiness to conduct provocations that cost innocent lives,” Panetta told a press conference at the end of his three-day visit, in which he expressed full commitment to the defence of South Korea and other Asian allies.
The US defence chief said US troop levels in the South would not be cut from the current 28,500 despite spending cutbacks at the Pentagon.
The ministers in their statement said they would advance combined readiness capabilities on the border islands and other areas near the sea border.
They reaffirmed “that any North Korean aggression or provocation is not to be tolerated”.
Panetta and Kim urged the North to “demonstrate its genuine will toward denuclearisation through concrete actions” — restating a condition set by the US and its allies before six-nation nuclear disarmament talks can resume.
The North quit the six-party nuclear talks in April 2009, a month before staging its second atomic weapons test.
It has since repeatedly said it wants to come back without preconditions to the negotiations, which group the two Koreas, the US, China, Russia and Japan.
Washington and its allies say Pyongyang must first take steps to show its sincerity, such as shutting down the UEP.
Panetta Thursday expressed scepticism about the outcome of talks held between the US and North Korea in Geneva this week, which are aimed at setting terms for the resumption of the full six-nation forum.