Flying over the island of Jikdo in the West Sea, where the US Forces Korea's bombing range is located, the B-1B was joined by South Korea's squadrons of F-15K and two KF-16 fighter jets as part of the two countries' annual defense drills, added the source. It then headed back to the Guam base.

"Through this exercise, the South Korean and US Air Forces demonstrated their strong deterrence against North Korea's nuclear and missile threats," the South Korean Air Force said in a statement.

The practice is designed to improve interoperability and combined tactical capabilities, as well as the ability to promptly deploy the strategic asset when needed, it added.

US Air Force officials pointed out the importance of such a combined training.

"Working and training side-by-side with our ROK partners increases our ability to integrate air operations more effectively in order to deter aggression in the region," Maj. Ryan Simpson, Pacific Air Forces bomber operations chief, said in a separate press release. "As an added bonus, the ROK offers an excellent training environment for increasing the capability of our bomber crews."

ROK is the acronym for South Korea's official name, the Republic of Korea.

The flight came hours after North Korea test-launched a missile from its eastern military base that apparently ended in failure.

The long-range, high-speed bomber is known to carry the largest conventional payload of both guided and unguided weapons in the US Air Force inventory.

The Donald Trump administration has ratcheted up pressure on an increasingly provocative Pyongyang.

The US Navy's nuclear-powered submarine USS Columbus (SSN-762) and aircraft carrier USS Carl Vinson also participated in the bilateral training. The US deployed two B-1Bs to Korea last week as well.

The North has responded sensitively to the deployment of such high-profile US military assets near its soil.

A day after the news of two B-1Bs' previous sortie to Korea, Pyongyang's state news agency KCNA described it as "aimed at making preemptive strikes at major targets" there.

"This proves clearly that the enemies' reckless military act to make a preemptive nuclear attack upon the DPRK (North Korea) has reached a very dangerous phase," it said, also referring to the temporary positioning of the Carl Vinson in the South. (Yonhap)