The bombers took off from the US Pacific territory of Guam, before entering South Korean airspace and conducting firing exercises over the East Sea and Yellow Sea, South Korea's military said.
Gen. John Hyten, head of U.S. Strategic Command, has stated that the EMP threat is real, but the military appears more focused on North Korea's ballistic missiles and nuclear warheads.
The North Korean party anniversary passed without any missile launches or nuclear tests, but officials are wary the North could engage in more provocations at any time, especially with China to begin its all-important Communist Party Congress on October 18.
"Thus far, the suspected North Korean actions are consistent with a desire to demonstrate a deterrent capability rather than a prelude to an unprovoked first-strike in cyberspace; however, North Korea linked actors are bold, have launched multiple cyber attacks created to demonstrate national strength and resolve, and have little concern for potential discovery and attribution of their operations", FireEye researchers said in a blog.
Hackers linked to North Korea reportedly targeted US electric power companies, according to a leading cybersecurity firm.
During Tuesday's drills, the USA bombers, which flew from the Andersen Air Force Base in Guam, staged simulated air-to-ground missile striking drills off the peninsula's east coast before flying across the country accompanied by the two South Korean fighters.
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Private electric companies in the USA were targeted by suspected North Korean hackers last month. Defense officials refused to comment Wednesday.
There is no evidence that the hacking attempts were successful, but FireEye assessed that the targeting of electric utilities could be related to increasing tensions between the US and North Korea, potentially foreshadowing a disruptive cyberattack. The aircraft then conducted similar simulated air to ground striking drills off the peninsula's west coast, according to South Korea's Defense Ministry.
North Korea has said Trump is "on a suicide mission" against it, with the country's foreign minister calling the US President "mentally deranged" in a speech at the United Nations last month.
North Korean dictator Kim Jong-Un inspects a military site in North Korea in this undated picture released by state media.
The training was part of a programme of "extended deterrence" against North Korea, it added.