North Korea is always ready for an additional nuclear test at any time, South Korea's defence ministry said on Monday.

"North Korea has a tunnel where it can conduct an additional nuclear test," Moon Sang-gyun, a spokesman at the South Korean defence ministry, told a news briefing.

South Korea's Yonhap News Agency reported earlier that North Korea has completed preparations for another nuclear test, citing South Korean government sources who said the North may use a previously unused tunnel at its mountainous test site.