SEOUL, South Korea — North Korean hackers stole a vast cache of data, including classified wartime contingency plans jointly drawn by the United States and South Korea, when they breached the computer network of the South Korean military last year, a South Korean lawmaker said Tuesday.

One of the plans included the South Korean military’s plan to remove the North Korean leader, Kim Jong-un, referred to as a “decapitation” plan, should war break out on the Korean Peninsula, the lawmaker, Rhee Cheol-hee, told reporters.

Mr. Rhee, a member of the governing Democratic Party who serves on the defense committee of the National Assembly, said he only recently learned of the scale of the North Korean hacking attack, which was first discovered in September last year.

It was not known whether any of the military’s top secrets were leaked, although Mr. Rhee said that nearly 300 lower-classification confidential documents were stolen. The military has not yet identified nearly 80 percent of the 235 gigabytes of leaked data, he said.