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 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.