South Korea was on alert Thursday for possible political change in its nuclear-armed neighbour following the disclosure that longtime North Korean leader Kim Jong-Il suffered a stroke, according to AFP.

Defence Minister Lee Sang-Hee, who told parliament Kim had undergone surgery but is recovering, said a military plan is being drawn up for any contingency.

President Lee Myung-Bak told security ministers and aides that "thorough preparations should be made to minimise confusion over changes in North Korea`s political circumstances."

South Korea is still technically at war with its hardline communist neighbour. The two forces face off across a heavily fortified border, with US troops backing up the South.

Kim "collapsed because of a brain problem and had surgery from which he is recovering," minister Lee was quoted by lawmakers as telling a closed-door session of the legislature`s defence committee.

Officials said earlier on condition of anonymity that Kim, 66, was thought to have undergone surgery, but this was the first confirmation.

Seoul officials believe Kim is still in charge of affairs in North Korea, one of the world`s most reclusive regimes .

But analysts fear the powerful military could assume more power in any post-Kim era and take a harder line on nuclear disarmament and cross-border ties.

The defence minister, quoted by lawmaker Yoo Seong-Min, said no unusual troop movements have been detected in the North and South Korea`s military is maintaining its customary alert level.

Asked if Seoul should revive a joint US-South Korean contingency plan to prepare for sudden political change in the North, Lee said relevant agencies were in talks toward a strategy "in preparation for a (possible) limited provocation or a full-scale war."

The intelligence agency told parliament Wednesday that Kim is still able to run the country and will recover. He is not fit enough for public activities but can speak without difficulty, they said.

Yonhap news agency quoted an unidentified top diplomatic source as saying there was no sign of any power vacuum in the North. "Key policy decisions are being made normally," the source said.

News of the stroke emerged after Kim failed to attend a parade on Tuesday marking the 60th anniversary of the founding of his country, in which he is officially accorded almost godlike status.

Lee Chul-Woo, secretary of parliament`s intelligence committee, said Kim is "recovering fast," can stand if supported and has no problems communicating.

He said the Seoul government has been aware of Kim`s health problems since mid-August. The North`s state media has not reported any public appearances by Kim since August 14, when he inspected an army unit.

Lee Chul-Woo said a collective leadership was more likely should there be a sudden transition, but if Kim`s condition improves, the succession would be determined later.

If the North`s powerful military secured more power in a post-Kim era, "it will change toward a tougher line," he said.

"If technocrats in the administration get more power, it will then move towards inter-Korean reconciliation."

Kim`s health has been the subject of intense speculation since he took over from his father, who died in 1994, in the communist world`s only dynastic succession. He has not publicly nominated any successor.

His illness comes amid deadlock in a six-nation nuclear disarmament deal and fears the North intends to restart its atomic weapons programme.

The North promised to shut down the programme after testing a nuclear device in October 2006 for the first time.

But it has halted work to disable its plutonium-producing plants, and says it will start repairing them again because of the deadlock over how to verify its nuclear disclosures.

North and South Korea have remained technically at war since their 1950-53 conflict ended in an armistice, not a peace treaty.