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 that while Washington was open to dialogue they would seek a new UN resolution on sanctions for Pyongyang.
“Our intention and that’s shared with the Republic of Korea is to secure the strongest possible (U.N. Security Council) resolution that includes new sanctions as quickly as possible.”
Shrugging off pressure, Pyongyang said attempts to prevent it becoming a nuclear power were futile.
North Korea carried out last week’s test despite sanctions and Seoul says the North is ready to carry out a new test at any time.
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.
Washington has made de-nuclearisation a condition for dialogue, something which Pyongyang refuses.