The South Korean military is investigating a computer hacking attack that may have stolen secret plans concerning military operations with the US.

South Korea Defence Ministry spokesman Won Tae-jae said the suspected hacking occurred late last month when a South Korean officer failed to remove a USB device when he switched a military computer from a restricted-access intranet to the internet.

The hacked information from the USB contained a summary of plans for military operations by South Korean and US troops in case of war on the Korean peninsula.

“The stolen documents were not a full text of the operational plans, but about an 11-page document used to brief military officials,” Won said.

The hackers used a Chinese IP address in the attack in which North Korea is suspected, according to