The US has deployed four F-22 stealth jets in South Korea, in a show of force following North Korea's recent nuclear tests and satellite launch.
The Raptors flew over the US military air base in Osan in Gyeonggi province, bordering North Korea, on Wednesday.
The move comes as the US and South Korea discuss possibly deploying a missile defence system in the South.
North Korea's actions have attracted international condemnation and calls for new UN sanctions.
Relations with the South have deteriorated, with Seoul suspending operations of the jointly-run Kaesong industrial park.
The US Raptors, one of the world's most advanced military aircraft, were flown into South Korea from their base in Okinawa in Japan.
South Korea' Yonhap news agency said the deployment of four stealth jets by the US at one time was rare.
It quoted military sources as saying the jets would remain in South Korea "for a while".
The US has some 28,000 military personnel based in South Korea.
In January the US flew a B-52 bomber over South Korea shortly after the North conducted a test of what it said was a hydrogen bomb.
The North then launched a rocket earlier this month, saying it was a satellite, but others believe it was a long-range missile test.