A South Korean official says North Korea is capable of conducting another nuclear test anytime at one of its unused tunnels at the country's main atomic test site.

The comments by Defense Ministry spokesman Moon Sang Gyun on Monday came three days after North Korea carried out its fifth atomic bomb test.

Moon refused to say what evidence pointed to another possible North Korea test, but said South Korea and U.S. intelligence authorities believe North Korea has the ability to detonate another atomic device anytime.

South Korea's Yonhap news agency, citing unidentified Seoul government sources, reported Monday that there is evidence showing the North has finished new test preparations at one previously unused tunnel in the Punggy-ri test site.

Yonhap didn't say how the sources got the information.