North Korean hackers may have gained access to war plans.

SEOUL, South Korea - Computer hackers who may be from North Korea have gained access to a secret U.S.-South Korean plan to defend the peninsula in case of war, the defense ministry said Dec. 18.

The hackers used an IP address in China to access some military data related to Operation Plan 5027, a spokesman told AFP.

"Authorities are trying to find whether North Korea was involved," he said, adding the leaked data contained crucial information such as slide and power point displays explaining the plan.

OPLAN 5027 was drawn up jointly by South Korea and the U.S., which stations 28,500 troops in the South. It allows for the dispatch of nearly 700,000 U.S. troops to the peninsula should a full-scale war break out.

The plan also sets wartime operational guidelines for the troops of the two countries. South Korea has technically remained at war with the North since the 1950-53 war ended in a truce, not a peace treaty.

"A probe is underway to figure out how much the leakage will affect our military plan," the spokesman said. "The officer concerned will be disciplined."

Chosun Ilbo newspaper said the officer with the Combined Forces Command had used an unsecured USB memory stick to download the plan.

South Korea believes the North has military personnel who specialize in overseas hacking and will set up its own military cyber command Jan. 1.

South Korea's spy chief has blamed North Korea's telecommunications ministry for cyber attacks that briefly crippled unclassified U.S. and South Korean government and commercial Web sites in July.