Gen. Jung Seung-jo, chairman of South Korea's Joint Chiefs of Staff, made the remarks during a parliamentary defense meeting as North Korea continued to make threats as it prepares to conduct its third nuclear test, Yonhap News Agency reported.
The North has issued a number of provocative statements since the U.N. Security Council last month approved a resolution tightening existing sanctions against it for a Dec. 12 long-range rocket firing. The North's first two nuclear tests were in 2006 and 2009.
On Tuesday, North Korea reportedly warned of "stronger" measures besides its third nuclear test in response to "hostile forces' nuclear-war moves" although it didn't elaborate.
When asked if the South Korean military was ready to engage in a pre-emptive strike to deter a nuclear attack, Jung said: "If [North Korea] shows a clear intent to use a nuclear weapon, it is better to get rid of it and go on a war, rather than being attacked," Yonhap reported.
The general said South Korea would make a decision on whether to destroy the North's nuclear test site after watching developing circumstances, the report said.
Jung didn't agree with speculation the North might test a hydrogen bomb, the report said. Instead, he said it is likely to be a "boosted fission weapon" or a smaller and more sophisticated bomb that can use either uranium or plutonium.
Critics have said the Dec. 12 long-range rocket firing was to test the North's intercontinental ballistic missile capability and that its success may mean it can place a small warhead on a missile that can reach as far away as the continental United States.
"Considering that the North conducted nuclear tests in 2006 and 2009, it has made considerable progress in miniaturizing the nuclear bomb," Jung was quoted as saying.
Earlier reports, citing satellite images, have talked of heightened activity near North Korea's Punggye-ri test site, but it was still not clear if any test would be an underground one.
On Wednesday in China, a close ally of North Korea, its Foreign Ministry spokeswoman Hua Chunying said her government opposes any moves that could create more tension on the Korean Peninsula, the official Xinhua News Agency reported.
She said Yang had told Kerry the Chinese side is committed to the achievement of denuclearization.
U.S. State Department spokeswoman Victoria Nuland has said Kerry and Yang talked of the countries staying "unified" to make clear to North Korea it must refrain from further provocation.
Kerry had held similar telephone talks with the foreign ministers of Japan and South Korea, and Nuland said the talk with the Chinese foreign minister was "remarkably similar" to those with the other two officials.