Two huge US strategic bombers flew over South Korea on Tuesday in a show of force against North Korea following Pyongyang's fifth and largest-ever nuclear test.
The supersonic B-1B Lancers reached airspace over the US Osan Air Base at Pyeongtaek, 64 kilometres (40 miles) south of Seoul, at around 10:00 am.
Each aircraft, which had flown from the US Pacific Command's Andersen Airforce base at Guam, was escorted by US and South Korean fighter jets.
"Today's demonstration provides just one example of the full range of military capabilities in the deep resources of this strong alliance to provide and strengthen extended deterrence," said General Vincent Brooks, US-South Korea Combined Forces Command.
"North Korea's nuclear test is a dangerous escalation and poses an unacceptable threat," said the general, adding the US has an "unshakable commitment" to defend allies in the region.
The US "will take necessary steps to do so, including operations like this one today, and the deployment of the Terminal High Altitude Air Defense (THAAD) battery to the Korean Peninsula," he said.
South Korea in July announced plans to deploy THAAD, a sophisticated US missile defence system, to counter growing nuclear and missile threats from the North.
But China, which believes the system's radar could also be used to track its own defences, objected strongly.
The plan has also met resistance from residents of the southern county of Seongju, the planned site.
South Korean opposition parties oppose the THAAD deployment, insisting it could escalate a regional arms race and hurt ties with China.
They also say, its usefulness against the North's military threats is doubtful.
Tuesday's overflight was "aimed at sending a clear warning to North Korea about its nuclear test, and containing further provocations," South Korea's defence ministry spokesman Moon Sang-Gyun told journalists.
Washington took similar actions after previous atomic tests.
In January, the US flew a B-52 Stratofortress strategic bomber over Osan Air Base after North Korea's fourth nuclear blast.
South Korea is home to around 28,500 US troops and numerous bases.