The South Korean and U.S. militaries flew 20 fighter jets over waters off South Korea's western coast Tuesday
A senior U.S. official warned of a strong response if North Korea conducts its first nuclear test in five years.
South Korea's Joint Chiefs of Staff said 16 South Korean planes, including F-35A stealth fighters, and four U.S. F-16s demonstrated their ability to respond quickly to North Korean provocations.
U.S. and South Korean officials say North Korea may soon conduct another nuclear test in Punggye-ri.
Deputy Secretary of State Wendy Sherman warned of a "swift and forceful" response if the North conducts another nuclear test.
"Nuclear tests would violate UNSC resolutions. Sherman said this after meeting with South Korea's Vice Foreign Minister Cho Hyun-dong.
The U.S. aircraft carrier Ronald Reagan concluded a three-day naval drill with South Korea in the Philippine Sea on Saturday, their first joint drill involving a carrier since November 2017.
A statement from the North Korean government-controlled media has not yet been made in response to Sunday's drill.