SEOUL » North Korea is capable of detonating another nuclear device anytime at one of its unused tunnels at the country’s main atomic test site, Seoul official said Monday, three days after the country carried out its fifth bomb explosion.

The North’s latest nuclear test was the most powerful to date and its claim to have used “standardized” warheads sparked worries it was making headway in its push to develop small and sophisticated warheads to be topped on missiles. Seoul, Washington and their allies subsequently vowed to apply more pressure and sanctions on Pyongyang.

After the test, the North’s nuclear weapons institute said it will take unspecified measures to further boost its nuclear capability, which analysts said hinted at a possible sixth nuclear test.

South Korea’s Defense Ministry spokesman Moon Sang Gyun said Monday that South Korea and U.S. intelligence authorities believe North Korea has the ability to detonate another atomic device anytime at the Punggye-ri test site, where the five previous atomic explosions took place.

Moon refused to say what specific evidence pointed to another possible North Korea test.

Yonhap news agency, citing unidentified Seoul government sources, reported Monday there were signs the North had finished test preparations in an unused tunnel. Yonhap did not elaborate.

South Korea’s top military intelligence officer, Kim Hwang Rok, said Friday that North Korea has two or three unused tunnels in the Punggy-ri site where it can conduct an additional test if it wants. Kim, director of the Korea Defense Intelligence Agency, made the remarks during a meeting with South Korea’s ruling party chief Lee Jung-hyun, according to party spokesman Yeom Dong-yeol.

South Korea has long avoided harsh rhetoric against North Korea and its leader Kim Jong Un but after Friday’s nuclear test, South Korean President Park Geun-hye said Kim’s “mental state is spiraling out of control” and that his government has “fanatic recklessness.”

South Korea’s military also started talking publicly about its military capabilities, revealing it has retaliation plans involving precision-strike missiles and special operations forces for direct attacks on the North’s leadership in the case of a North Korean nuclear attack.

Yonhap, citing an unidentified military source, said Sunday that the plan would turn areas in Pyongyang, where the North’s war commanders were likely to hide, into ashes and “eliminate those places from the map permanently.” South Korea’s Defense Ministry said it would not comment on the report.

Bad weather Monday also delayed a U.S. plan for at least 24 hours to send warplanes from Guam to South Korea in a show of force, as it has done in the past after major provocations by North Korea.

“The U.S. stands resolutely with the Republic of Korea and continues its ironclad support,” Christopher Bush, a spokesman at the U.S. military command in South Korea.

North Korea’s pursuit of missiles and nuclear weapons is one of the most intractable foreign policy problems for the U.S. and South Korea. Diplomacy has so far failed. Six-nation negotiations on dismantling North Korea’s nuclear program in exchange for aid were last held in late 2008 and fell apart in early 2009.

The Korean Peninsula remains technically at war, as the 1950-53 Korean War ended in an armistice, not a peace treaty. The United States has about 28,500 troops in South Korea.