North Korea has kept up preparations for a new nuclear test after having carried out previous launches in 2006 and 2009, South Korea's Defense Minister Kim Kwan-Jin told a news conference Wednesday.

"In fact, North Korea has been preparing for this for quite a long time," Kim told a news conference with Pentagon chief Leon Panetta.

"And when the time comes for a political decision, it may in fact resort to this third nuclear test," he said.

Kim endorsed efforts to persuade Pyongyang to resume six-nation talks on halting its drive to build nuclear weapons -- the discussions have been frozen since December 2008 -- and described the regime under new leader Kim Jong-Un as "quite stable."

The new leader has tried to carry out economic reforms but the effect remained unclear, Kim said.

"He seems to be making attempts to bringing a better life to his people, but the likelihood of success ...it's yet to be seen," he said through an interpreter.

But Kim Jong-Un also appeared to be following his late father's approach of putting the military first, before trying to satisfy the needs of the country's impoverished population, the minister said.

His youth was also another factor to take into account, he added.

"He is still young, meaning that he may be a lot more aggressive compared to old people, because he's still young," Kim said.

Panetta, former director of the Central Intelligence Agency, said the issue of how the new ruler of North Korea would behave remained an open question.

"I think the bottom line is we still don't know whether or not he will simply follow in the steps of his father or whether he represents a different kind of leadership for the future," the defense secretary said.

Panetta condemned North Korea's "provocative" stance and said Washington "reaffirmed its firm commitment to the security of the Korean Peninsula by maintaining the current level of US forces in Korea."

The United States retains 28,500 troops in South Korea as well as missile defenses in the area and a nuclear "umbrella" in case of an attack from the North.

A recent North Korean threat to attack the South did not materialize after Seoul stopped activists from launching anti-Pyongyang propaganda leaflets across the heavily militarized border.