North Korea has threatened to "wipe out" South Korea's government in a furious response a day after a Seoul official said the North "must disappear soon", in an escalation of rhetoric between the rivals.
The North's powerful National Defence Commission called the South Korean comments an "intolerable" provocation that showed the South wants to take over the North.
It said in a statement carried by state media that Pyongyang will launch "all-out ... merciless" strikes to "wipe out every last person" in South Korean president Park Geun-hye's government.
South Korean Defence Ministry spokesman Kim Min-seok said North Korea was not a real country and existed for the benefit of only one person - a reference to its leader, Kim Jong Un. He said the North has no human rights or public freedoms.
The comments followed a series of slurs by North Korea against the leaders of South Korea and the United States. North Korea's media likened Ms Park to an "old prostitute" and president Barack Obama to a monkey in recent dispatches.
Seoul's Defence Ministry said the North's military had not undertaken any suspicious activities. An official said South Korea is always ready to repel any provocation by the North.
South Korea has been highly critical of its rival's nuclear and missile programmes, including recent rocket and missile launches and apparent preparations for a fourth nuclear test, but yesterday's comments from Seoul were stronger than usual.
The South tries to avoid talking publicly about anything that could be interpreted as a collapse of the North Korean government because of worries that the North would raise tensions.
North Korea has been ramping up its rhetoric against the South and Washington since Mr Obama and Ms Park met in Seoul last month. During the visit, Mr Obama said it might be time to consider further sanctions against Pyongyang and that the US will not hesitate to use its military might to defend its allies.
South Korea has called the North's verbal insults against Ms Park immoral and unacceptable. The US State Department described the North's racist slurs against Mr Obama as "disgusting".
The North Korean government-run Uriminzokkiri website warned that Kim Min-seok would pay a high price for his comments. "We will not sit idle by while this mad dog keeps ... barking noisily," the website said.