It was part of a "regular deployment training" aimed at enhancing the capability of implementing the "extended deterrence" against the North, the JCS said. It involved a pair of B-1B Lancers staging a simulated air-to-ground missile firing with two South Korean F-15K jets over the Sea of Japan which then flew across the southern portion of the Korean peninsula.
North Korea warned last month it could shoot down US bombers at its discretion amid heightened tensions in the wake of Pyongyang's sixth ever nuclear test.
North Korean leader Kim Jong Un gives field guidance at the Sci-Tech Complex, in this undated photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang October 28, 2015.
While the officials said the exercises are long scheduled and not in response to recent tensions, similar exercises have prompted strong condemnation from North Korea in the past. The South has taken an increasingly aggressive stance toward the North's belligerence amid back-and-forth threats of war between North Korea and U.S. President Donald Trump.
The U.S. Pacific Air Forces (PACAF) said in a statement this was the first nighttime American B-1 bomber drill conducted with the two U.S. allies in the region.
There are also serious concerns about North Korea's cyberwarfare capabilities, but the USA has been carrying out operations of its own to counter North Korean cyber operations in Northeast Asia and elsewhere. Kim is the third generation of his family to rule North Korea.
Private electric companies in the USA were targeted by suspected North Korean hackers last month.
The South's Yonhap news agency quoted Lee as saying that 235 gigabytes of military documents were taken.
The F-15 escorts are notable "because combined operations require consent from South Korea, so this does not represent USA unilateral action", according to Pinkston. Other stolen data included contingency plans for South Korean special forces and information on military facilities and power plants, he said.
There is no evidence that the attacks were successful, and cybersecurity experts believe that North Korea lacks the ability to disrupt the power grid.
North Korea's possession of secret war plans would require a major overhaul of how South Korea and its ally Washington would respond if there's another war on the Korean Peninsula.
North Korea has yet to comment on either the bombing drills or the hacking claims.