The information, that included wartime contingency plans drawn up by the US and South Korea, was from the country's defence ministry, according to Rhee Cheol-hee, a South Korean lawmaker.
"FireEye devices detected and stopped spear phishing emails sent on September 22, 2017, to USA electric companies by known cyber threat actors likely affiliated with the North Korean government", the infosec outfit reported on Tuesday.
Joined by South Korean F-15K fighters, the B-1Bs simulated air to ground missile firing over the East Sea.
"While I would not characterize them as the best in the world, they are among the best in the world and the best organized", Army Gen. Vincent Brooks, commander of U.S. Forces Korea, told the Senate past year. Defense officials refused to comment when reached by other media.
While North Korea was not able to hack US power companies, there are concerns among some of observers that the regime may attempt to damage the power grid by detonating a nuclear device at a high altitude above the USA, triggering an electromagnetic pulse (EMP).
Meanwhile, the USS Tucson, a nuclear-powered attack submarine of the U.S. Navy, has arrived at the Chinhae base in South Korea, according to the U.S. Pacific Command.
A spokesman from South Korea's Defense Ministry declined to comment, saying the information is classified.
The drills were conducted not long after Lee broke the news about the alleged cyberattacks to reporters.
A pre-emptive strike against Pyongyang's leadership would be hard to undertake, but it's widely seen as the most realistic of the limited military options Seoul has to deny a nuclear attack from its rival.
In an obvious show of force over the continued missile activities of North Korea, led by Kim Jong Un, the USA military has flown two strategic bombers over the Korean peninsula. But Kim, the third generation of his family to rule, is officially revered in the North, and any suggestion of removing him from power is taken extremely seriously in Pyongyang.
"They likely remain committed to pursuing targets in the energy sector, especially in South Korea and among the US and its allies, as a means of deterring potential war or sowing disorder during a time of armed conflict".
The training was part of a programme of "extended deterrence" against North Korea, it added.