He pointed out the allies have regularly conducted such a joint air training.

A source said the bombers, a key strategic asset of the US forces in the Asia-Pacific region, will carry out mock bombing practice at the Pilsung Range in the eastern province of Gangwon.

The US Pacific Air Forces would not talk in public about the issue apparently before the end of the bombers' mission.

"For operational security reasons, we will not discuss potential operations we plan to conduct with our allies," a PACAF official said. 

The sortie of the US bombers based in Guam comes after the reported death of a US college student released from North Korea in a coma after 17 months of detention.

Otto Warmbier, a 22-year-old University of Virginia student, died at a Cincinnati hospital on Monday (local time), according to his family.

The B-1B's flight here also follows controversial remarks by Moon Chung-in, a special adviser to South Korean President Moon Jae-in on unification, security and foreign affairs, that Seoul may decrease the deployment of US strategic assets here and the number of the allies' combined military drills if the North freezes its nuclear and missile development.

The presidential office dismissed it as the scholar's personal view, not the government's official position.

The US sent B-1B bombers to Korea on May 29, hours after North Korea test-fired a ballistic missile.

Meanwhile, a major US warship arrived in South Korea on Tuesday in a demonstration of the naval partnership between the two sides.

The Arleigh Burke-class guided-missile destroyer USS Dewey (DDG 105) made a routine port call to Jeju Island, according to the US Navy.

"This visit is yet another example of our longstanding partnership with the (South Korean) Navy," said Capt. Glenn Leverette, the deputy commander of US Naval Forces Korea.

The Dewey, which is on patrol in the US 7th Fleet area of operations in support of security and stability in the Indo-Asia-Pacific region, is also going to join a scheduled joint exercise with South Korean and Canadian naval ships.

"This visit is important because with each engagement, we improve our cooperation and enhance our combined defensive capabilities," said Cdr. Anthony Webber, the Dewey's commanding officer. (Yonhap)