The US and South Korean defence chiefs vowed Friday to raise combat-readiness against North Korean attacks following two deadly incidents last year, saying aggression "is not to be tolerated".

Leon Panetta and his counterpart Kim Kwan-Jin also described the North's uranium enrichment programme revealed last November as a "grave threat" that gave it a second path to develop atomic weapons.

Kim said there was a high possibility of further provocation next year as Pyongyang's regime works to put a leadership succession plan in place.

The ministers made their commitments in a joint statement at the end of security talks.

Inter-Korean ties have been tense since Seoul accused its neighbour of torpedoing a South Korean warship in March 2010 near the Yellow Sea border with the loss of 46 lives.

The North denied sinking the ship 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 defense secretary said US troop levels in the South would not be reduced from the current 28,500 despite spending cutbacks at the Pentagon.

The ministers in their statement said they would advance combined readiness capabilities on islands and other areas near the disputed Yellow Sea border, a frequent flashpoint.

Any "North Korean aggression or provocation is not to be tolerated", they reaffirmed.

"Next year, I believe the possibility that NK conducts a further provocation is high," the South's defence minister Kim said, noting a major anniversary in 2012 and the ongoing succession process.

The allies would "respond very strongly" to any new provocation, he said, adding a joint plan for such a response would be completed this year.

The North vows to become a "great, powerful and prosperous nation" by next year, the 100th anniversary of the birth of founder Kim Il-Sung.

Kim's son and current leader Kim Jong-Il is grooming his own son Jong-Un to take over eventually.

Minister Kim said the United States had agreed immediately to provide "an overwhelming number of forces" during crises. Asked about the comment, Panetta's spokesman George Little said he "wouldn't speculate about hypotheticals".

Panetta and Kim urged the North to "demonstrate its genuine will toward denuclearisation through concrete actions" -- restating a condition set by the United States and its allies before six-nation nuclear disarmament talks can resume.

The North quit the six-party 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 United States, China, Russia and Japan.

Washington and its allies say Pyongyang must first take steps to show its sincerity, such as shutting down the uranium enrichment programme.

Spelling out steps required, Panetta said the North must stop testing and developing weapons, stop enrichment activity and re-admit UN nuclear inspectors.

Panetta Thursday had expressed scepticism about the outcome of talks held between the United States and North Korea in Geneva this week, which are aimed at setting terms for the resumption of the full six-nation forum.

"The word scepticism would be in order at this time as to what may or may not happen in those discussions," he told reporters.