Two US B-1 bombers have flown over South Korea. The nuclear capable aircraft are a clear signal to North Korea after it carried out a nuclear test last week.
China has called for calm, saying all sides should avoid escalating tensions.
The US special envoy on North Korea, said while Washington was open to dialogue they would seek a new UN resolution on sanctions for Pyongyang.
“The United States has an unshakable commitment to defend allies in the region and will take necessary steps to do so, including operations like the one you observed today and also the deployment of the Terminal High Altitude Area Defence battery, the THAAD (Terminal High Altitude Area Defence) battery, to the Republic of Korea,” US Forces Korea Commander General, Vincent K. Brooks said.
Washington has made de-nuclearisation a condition for dialogue, something which Pyongyang refuses.
North Korea carried out last week’s test, despite sanctions. Seoul says the North are ready to carry out a new test at any time.
Shrugging off pressure, Pyongyang said attempts to prevent it becoming a nuclear power were futile.
Six party talks between the US, Russia, South Korea, China and North Korea on ending the North’s nuclear programme have been stalled since 2008.