North Korea opposes war, but if South Korea chooses military confrontation or makes a preemptive strike, then the North's nuclear forces will have to attack, the powerful sister of leader Kim Jong Un said on Tuesday.

Kim Yo Jong, a senior official in the government and ruling party, said it was a "very big mistake" for South Korea's minister of defence to make recent remarks discussing attacks on the North, state news agency KCNA reported.

South Korean Defence Minister Suh Wook said on Friday that his country's military has a variety of missiles with significantly improved firing range, accuracy and power, with "the ability to accurately and quickly hit any target in North Korea."

North Korea has test-fired a range of increasingly powerful missiles this year, and officials in Seoul and Washington fear it may be preparing to resume testing nuclear weapons for the first time since 2017 amid stalled negotiations.

Kim and another North Korean official issued earlier statements on Sunday condemning those remarks, and warned that Pyongyang would destroy major targets in Seoul if the South takes any "dangerous military action" such as a preemptive strike.

In her second statement, on Tuesday, Kim said Pyongyang opposes war, which would leave the peninsula in ruins, and does not view South Korea as its principal enemy.

"In other words, it means that unless the south Korean army takes any military action against our state, it will not be regarded as a target of our attack," she said.

"But if south Korea, for any reason - whether or not it is blinded by misjudgment - opts for such military action as 'preemptive strike' touted by (Suh Wook), the situation will change," Kim added. "In that case, south Korea itself will become a target."

If the South Korean military violates even an inch (2.5 cm) of North Korea territory, it will face an "unimaginably terrible disaster" and the North's nuclear combat force will have to inevitably carry out its duty, she said.

"This is not just a threat. This is a detailed explanation of our reaction to possible reckless military action by south Korea," Kim said, noting that the South can avoid this fate by dropping any "fantastic daydream" of launching a preemptive attack on a nuclear-armed state.