South Korean officials believe that North Korean hackers stole classified U.S.-South Korea military documents, including a top secret plan prepared for potential conflict on the Korean Peninsula.

Multiple South Korean media outlets reported Tuesday that Seoul’s defense ministry disclosed in communications with a South Korean lawmaker that the military documents had been stolen.

According to a report in the Chosun Ilbo newspaper, the defense ministry said that hackers “presumed to be North Koreans” stole 235 gigabytes of documents from South Korean military networks in August and September of 2016.

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The ministry disclosed the leak in documents submitted to Rhee Cheol-hee, a member of the country's legislature, in response to a freedom of information request.

Among the documents reportedly harvested was OPLAN 5015, a top secret U.S.-South Korea military strategy for full-scale conflict on the Korean Peninsula. Hackers also pilfered U.S. and South Korea personnel reports, minutes from meetings about joint military drills between the U.S. and South Korea, and data on South Korean military installations and power plants, according to news outlets.

Lee told South Korea’s Yonhap News Agency that much of the stolen data has not been identified.

“The Ministry of National Defense has yet to find out about the content of 182 gigabytes of the total (stolen) data,” Lee said.

Seoul had previously said that North Korea was likely responsible for the hack of the defense ministry’s internet but never revealed that any information had been stolen.

The revelation comes amid heightened tensions over North Korea’s nuclear and ballistic missile tests.

The U.S. military has roughly 28,000 troops stationed in South Korea.