"North Korea is not having a responsible attitude yet," Kim Hwang-Sik said while presiding over an annual defence meeting of top military commanders, ministers and presidential aides.

"Seem the situation in North Korea, there is a chance of its military aggravation again," Yonhap news agency quoted Kim as saying. "So (the South's military) should be fully prepared."

Tensions have been elevated since the North's bombardment of a South Korean border island last November, which killed four people including civilians.

Seoul also accuses Pyongyang of sinking one of its warships last March with the loss of 46 lives, a charge it denies.

Military talks aimed at improving relations broke down last week, and the North said there was no need for further dialogue with "traitors" in the South.

Satellite photos made public this week show that the North has apparently completed work on a new and larger launch site for long-range missiles.

Admiral Robert Willard, head of the US Pacific Command, said in Washington Thursday there are no signs of an upcoming launch.

But he expressed fears of some sort of new "provocation" within months.

Many analysts and officials believe last year's attacks were aimed at burnishing the military credentials of Kim Jong-Un, the youngest son and heir apparent to elderly leader Kim Jong-Il.

Pointing to the succession process, Willard said: "We may very well be facing the next provocation in months and not years."