The US sent a B-52 bomber on a low-level flight in South Korean airspace in a show of force following North Korea's nuclear test.
The B-52 was joined by two fighter planes - a US F-16 and a South Korean F-1 - as it flew over Osan Air Base near Seoul, according to a US military statement.
The flight was "in response to recent provocative action by North Korea", the statement said.
The United States remains steadfast in its commitment to the defense of the Republic of Korea (South Korea) and to maintaining stability on the Korean Peninsula, to include extended deterrence provided by our conventional forces and our nuclear umbrella.
The B-52 is based in Guam and is capable of carrying nuclear weapons.
North Korea conducted a nuclear test - its fourth - on Wednesday last week, angering the US and China.
Kim Jong Un, the North Korean leader, has maintained the test was of a hydrogen bomb and said it was a self-defensive step against a US threat of nuclear war.
Experts believe the North's nuclear test, which produced a seismic tremor of 5.1, was too small to be a proper hydrogen bomb test.