The strategic bombers from Guam conducted a joint "air interdiction operation" with four F-15K fighter jets, said South Korea's Air Force, source from the Yonhap.
It came in response to North Korea's repeated ballistic missile launches and nuclear weapons development.
"Our Air Force maintains the precision-strike capability and readiness to burn the enemy's missile base and its key facilities, including its leadership stronghold to the ground," South Korea's Air Force Operations Commander Lt. Gen. Won In-cheol said in a statement. "If the enemy provokes, we will respond immediately and retaliate powerfully."
The U.S. 7th Air Force said the combined training was aimed at demonstrating the "ironclad U.S. commitment" to the defense of the ally.
"North Korea remains the most urgent threat to regional stability," said Gen. Terrence J. O'Shaughnessy, Pacific Air Forces commander. "Diplomacy remains the lead; however, we have a responsibility to our allies and our nation to showcase our unwavering commitment while planning for the worst-case scenario. If called upon, we are ready to respond with rapid, lethal and overwhelming force at a time and place of our choosing."
The North test fired the ICBM late Friday night, the communist nation's second ICBM launch in less than a month.
The U.S. sent two B-1Bs here in early July for a joint exercise with South Korea's Air Force, days after the North's previous ICBM launch.