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.
It did not elaborate on what activities had been detected at the Punggye-ri nuclear test site near the North's northeastern shore, the location of its five nuclear explosions.
North Korea said on Sunday a push for further sanctions following its latest nuclear test was "laughable", and vowed to continue to strengthen its nuclear power, Reuters reported.
South Korea said on Saturday that the latest test showed North Korea's nuclear capability was expanding fast and that North Korean leader Kim Jong Un was unwilling to alter course.
The UN Security Council denounced North Korea's decision to carry out the test and said it would begin work immediately on a resolution.