The United States flew two B-1B bombers over the Korean Peninsula on Thursday, its Air Force said, ahead of US President Donald Trump's planned visit to Asia.

The pair of bombers conducted sequenced bilateral missions with South Korean fighters and Japanese fighters, the US Pacific Air Forces said in a statement.

"The two B-1B Lancers took off from Andersen Air Force Base, Guam, and transited south of Korea and west of Japan to integrate with Koku Jieitai (Japan Air Self-Defense Force) fighters," it said. "The Lancers then transited overland to Korea to integrate with Republic of Korea fighters in the Yellow Sea. Upon completion of bilateral integration, the aircraft returned to their respective home stations."

The mission was planned in advance and not in response to any current event, it added.

Trump is set to embark on his first official visit to Asia on Friday. The White House has said the trip will largely focus on ways to counter the threat posed by North Korea's nuclear and ballistic missile programs.

The 12-day trip will include stops in South Korea, Japan, and China.

South Korea's military also said that the US bombers conducted simulated air-to-ground bombing drills over the Pilseung shooting range in Gangwon Province. They are known to have been escorted by two South Korean F-16K fighter jets.

"B-1B bombers have flown over the Korean Peninsula once or twice per month," a military source said. "The move is part of efforts to enhance rotational deployments of US strategic assets to curb North Korea's nuclear and missile threats.

In Pyongyang, North Korea slammed the exercise as an attempt to "threaten and blackmail" the regime.

"The frenzied US scheme of threats and blackmail, which aims to crush our republic with nuclear (weapons) continues in November as it did in October," the state-run Korean Central News Agency said hours before the US announcement.

The bombers flew over Okinawa, Japan, and then the southern South Korean island of Jeju, before changing direction and heading toward the East Sea, it said.

Under the escort of South Korean fighter jets, the US airmen practiced dropping nuclear bombs on "key" North Korean targets set up on a South Korean shooting range, it added.

The agency argued that what makes the situation worse is the simultaneous deployment of three US aircraft carrier strike groups in waters near the Korean Peninsula.

"The US may make frantic last-minute efforts to block our powerful advances by shoving nuclear strategic assets at us, but our army and people are not ones to be startled," it said.

The US has recently conducted similar flyovers near the Korean Peninsula to deter North Korea from further nuclear and missile tests.

In July, the communist regime launched two long-range missiles potentially capable of reaching the mainland US In September, it conducted its sixth and most powerful nuclear detonation.

At the Security Consultative Meeting last Saturday, the defense chiefs of South Korea and the US reaffirmed the allies' plan to "enhance rotational deployments of US strategic assets in and around the peninsula."

For South Korea, US strategic assets usually mean such high-profile weapon systems as bomber jets, aircraft carriers and nuclear-powered submarines. (Yonhap)