South Korea threatens to reduce the North Korean capital to ashes if it shows any signs of mounting a nuclear attack.
North Korea is ready to conduct another nuclear test, South Korea's Defence Ministry said on Monday, three days after Pyongyang carried out a fifth such test that drew international condemnation.
Pyongyang set off its most powerful nuclear blast to date on Friday, saying it had mastered the ability to mount a warhead on a ballistic missile, ratcheting up a threat that its rivals and the United Nations have been powerless to contain.
"Assessment by South Korean and US intelligence is that the North is always ready for an additional nuclear test in the Punggye-ri area," South Korean Defence Ministry spokesman Moon Sang-gyun told a news briefing.
Punggye-ri, near the northeastern coast, was the site of the North's five previous nuclear explosions.
North Korea has a tunnel where it can conduct an additional nuclear test, Moon said.
A US special envoy for the isolated state, Sung Kim, will travel to Seoul on Monday after discussing cooperation among neighbouring countries in Tokyo in the wake of the North's latest nuclear test.
Kim met with Japanese officials on Sunday and said the United States may launch unilateral sanctions against North Korea, echoing comments by US President Barack Obama on Friday in the wake of the test.
A push for further sanctions was "laughable", North Korea said on Sunday, vowing to continue to strengthen its nuclear power.
"South Korea has already developed a plan to annihilate the North Korean capital of Pyongyang through intensive bombing in case the North shows any signs of a nuclear attack," South Korea's Yonhap News Agency said, quoting a military source.
"In other words, the North's capital city will be reduced to ashes and removed from the map."
South Korea's military put the force of Friday's blast at 10 kilotonnes, but a US expert said the highest estimates of seismic magnitude suggested a yield of 20 to 30 kilotonnes.
The test showed North Korea's nuclear capability was expanding fast and its leader Kim Jong Un was unwilling to change course, South Korea said on Saturday.
The UN Security Council denounced the test and said it would begin work immediately on a resolution. The United States, Britain and France pushed for the 15-member body to impose new sanctions.
It was not clear whether the Security Council can reach consensus on imposing fresh sanctions any time soon. China and Russia have previously vetoed sanctions.
Both countries did join sanctions in March, following the North's January nuclear test.
North Korea repeated its demand for recognition as a "legitimate" nuclear-armed state on Sunday. It said its missile and nuclear tests are necessary to counter "a US nuclear threat to its independence".
Russian Foreign Minister Sergei Lavrov said on Saturday called for a "creative" response.
The Global Times, run by the Chinese Communist Party, rejected the suggestion by the United States that Beijing was responsible for the North's pursuit of nuclear arms. It said the United States was "the root cause" of the issue.
"China is not capable of persuading North Korea to give up nuclear development, because China's efforts are not supported by the others," it said in an editorial on Monday. "Washington has been refusing to sign a peace treaty with Pyongyang."