Two US bombers departed from Andersen Air Force Base in Guam on Monday.
"The U.S. conducts Continuous Bomber Presence operations as part of a routine, forward deployed capability supporting regional security and our allies in the Indo-Asia-Pacific region," PACAF spokesperson Lt. Col. Lori Hodge was quoted as saying by Yonhap news agency.
The bombers flight triggered Pyongyang's sharp reaction as it took place a day after Seoul and Washington finished Key Resolve drills, lasting for two months.
The situation on the Korean peninsula aggravated in recent months as North Korea carried out a number of missile launches and nuclear tests.
The most recent missile test allegedly took place on Saturday from a site north of Pyongyang. Japan, South Korea and the United States said the test appeared to have failed as the missile broke up within North Korean territory.