Military officials cited in local media reports said the highly sensitive information was linked to Operation Plan 5027, a covert U.S.-South Korean plan to defend the peninsula in the event of war.
"Authorities are trying to find whether North Korea was involved," an unnamed defense spokesman was quoted saying in a report of The Korea Times. "A probe is under way to determine how much the leakage will affect our military plan," the official added.
Reports suggest that details of the defense plan, nicknamed Oplan 5027, may have been stolen by North Korean hackers last month after a South Korean officer used an insecure USB memory stick to download it from a restricted-access Internet provider.
The Chosun Ilbo newspaper reported that hackers used a Chinese IP address or Internet address. The North's involvement was not immediately confirmed.
Still, according to the same newspaper the covert plan outlines troop deployments, a list of North Korean targets, amphibious landing scenarios and how to establish a post-war occupation.
What's more, the plan is said to involve the deployment of up to 700,000 U.S. troops in the case of an all-out war on the peninsula.
"As a matter of policy, we do not comment on operational planning or intelligence matters, nor would we confirm details pertaining to any security investigation," said David Oten, a spokesman for the U.S. military stationed in South Korea.
An estimated 28,500 U.S. troops are already based in the south of the peninsula to deter any potential aggression from North Korea.
Although the Korean War ended in 1953, both countries have failed to sign a peace agreement, remaining divided by one of the world's most fortified borders.
Revelations that sensitive defense information may have found its way to hostile hands has sparked strong debate within South Korea.
It is the second cyberattack since July, when hackers caused Web outages at the White House and South Korea's presidential Blue House.
At the time, intelligence officials said the IP address from which the attack was launched was traced back to North Korea's Ministry of Post and Telecommunications that had leased the IP address from China.
Facing the military might of the United States, North Korea is seen as taking keen interest in recent years in information technology. Media reports have estimated that Pyongyang has drafted between 500 and 1,000 hacking experts.