TOKYO — North Korean hackers stole a huge trove of classified U.S. and South Korean military documents last year, including a plan to “decapitate” the leadership in Pyongyang in the event of war, a lawmaker in Seoul said Tuesday.

The purported revelations come at a time of heightened tensions over North Korea. President Trump recently said that “only one thing will work” when it comes to Pyongyang, hinting that he thinks diplomatic efforts are proving futile and military action may be necessary.

The defense minister in Japan, a close military ally of the United States, said Tuesday that Trump might take such action against North Korea as soon as next month.

In Seoul, Rhee Cheol-hee, a lawmaker in the ruling Democratic Party and a member of the parliamentary national defense committee, said North Korean hackers broke into the Defense Integrated Data Center in September last year to steal secret files, including American and South Korean “operational plans” for wartime action. The data center is the main headquarters of South Korea’s defense network.

According to Rhee, the stolen documents included OPLAN 5015, a plan drafted two years ago for dealing with full-blown war with North Korea and said to include procedures to “decapitate” the North Korean leadership. He said the cache also included OPLAN 3100, outlining the military response to infiltration by North Korean commandos or another local provocation, as well as a contingency plan in case of a sudden change in North Korea.

Pentagon spokesman Army Col. Robert Manning said Tuesday he was aware of media reports of the breach but would not say if sensitive operation documents were exposed. “We are confident in the security of our operations plans,” Manning said.

While the two Koreas have technically been on a war footing since the Korean War ended in an armistice in 1953, anything that suggests the death or ouster of North Korea’s leader, or his assassination, is tantamount to heresy in the North, where the ruling Kims are treated like gods.