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.
In her comments, Ms Park said: “If there is any provocation against South Korea and its people, there should be a strong response in initial combat without any political considerations,”
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.
South Korean Foreign Minister Yun Byung-se is expected to meet US Secretary of State John Kerry this week in Washington to discuss North Korea, Yonhap news agency reports.
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.