South Korea has promised a "strong response" to North Korean aggression, amid high tensions on the peninsula.
Speaking to defence officials on Monday, President Park Geun-hye said that she took the series of threats from Pyongyang "very seriously". North Korea said on Saturday that it was entering a "state of war" with South Korea.
On Sunday, the US sent stealth fighters to South Korea, as North Korea pledged to build up its nuclear arsenal. "If there is any provocation against South Korea and its people, there should be a strong response in initial combat without any political considerations," Ms Park said.
In recent days North Korea has issued multiple warnings of attacks on US and South Korean targets - to which the US has responded with an apparent show of military hardware.
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.
The US flew F-22 planes from Japan to South Korea's Osan Air base on Sunday, as part of ongoing joint military exercises with South Korea, 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".
It is not the first time F-22s have been used drills with South Korea, but the move came as North Korea's Central Committee held a rare high-level meeting on Sunday.
The committee described nuclear weapons as "the nation's life" and vowed to further develop its nuclear programme, state-run news agency KCNA said. "Only when the nuclear shield for self-defence is held fast, will it be possible to shatter the US imperialists' ambition for annexing the Korean Peninsula by force," the report added.
North Korea's Supreme People's Assembly - the rubber-stamp parliament - is also due to convene on Monday for a day-long annual session. While the group normally focuses on making economic decisions, this meeting will be keenly watched given the current high tension.
Few think the North - which last week cut a military hotline which was the last official direct link with Seoul - would risk full-blown conflict. But in recent years there have been deadly incidents such as the sinking of a South Korea warship (in which Pyongyang denies any role) and the shelling of a South Korean island. However the jointly-run Kaesong industrial park, which is located within North Korea's borders, remains in operation.
Workers from South Korea were crossing into the park - which is a key money-maker for North Korea - as normal on Monday, reports said.