The US says it sent stealth fighters to South Korea on Sunday, as the North pledged to build up its nuclear arsenal at a meeting of top leaders.The F-22 fighters were deployed as part of ongoing joint military exercises with South Korea, the US said.
In recent days B-52 bombers and stealth planes have also flown over the area, amid fiery rhetoric from Pyongyang.
North Korea said on Saturday that it was entering a "state of war" with the South.
The communist state has been angered both by UN sanctions imposed after its nuclear test in February and the joint US-South Korea annual military drills.
In recent days it has issued multiple warnings of attacks on US and South Korean targets - to which the US has responded with the various aircraft flights.
The US F-22 planes were flown from Japan to South Korea's Osan Air base on Sunday, officials said.
"[North Korea] will achieve nothing by threats or provocations, which will only further isolate North Korea and undermine international efforts to ensure peace and stability in Northeast Asia," the US military command in South Korea said in a statement reported by Reuters news agency.
In March, the US deployed both B-2 and B-52 planes, which have nuclear capabilities, over South Korea. It said this demonstrated its "capability... to provide extended deterrence to [its] allies in the Asia-Pacific region".
At a rare high-level meeting on Sunday, North Korea's Central Committee described nuclear weapons as "the nation's life" and vowed to further develop its nuclear programme, state-run news agency KCNA said.