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

Pyongyang on Friday set off its most powerful nuclear blast to date, saying it had mastered the ability to mount a warhead on a ballistic missile and ratcheting up a threat that its rivals and the United Nations have been powerless to contain.

"Indications have been gathered that the North has completed preparations to conduct a nuclear test at any time in the third tunnel that has not been used previously," Yonhap quoted one of the government sources as saying.