SEOUL (Reuters) - North Korea has completed preparations for another nuclear test, South Korea's Yonhap News Agency reported on 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.

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.

A U.S. special envoy for the isolated state, Sung Kim, will travel to Seoul on Monday after discussing with Japanese officials in Tokyo cooperation among neighboring countries in the wake of the North's nuclear test.

Kim met with Japanese officials on Sunday and said the United States may launch unilateral sanctions against North Korea, echoing comments by U.S. President Barack Obama on Friday in the wake of the test.

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.

South Korea's military put the force of Friday's blast at 10 kilotonnes, which would be the North's most powerful blast to date, but a U.S. expert said the highest estimates of seismic magnitude suggested a yield of 20 to 30 kilotonnes.

The U.N. Security Council denounced North Korea's decision to carry out the test and said it would begin work immediately on a resolution. The United States, Britain and France pushed for the 15-member body to impose new sanctions.

(Reporting by Ju-min Park and Jack Kim; Editing by Peter Cooney; Editing by Richard Pullin)