North Korea said on Sunday a push for further sanctions following its fifth and biggest nuclear test was "laughable", and vowed to continue to strengthen its nuclear power. The isolated state on Friday set off its most powerful nuclear explosion to date, saying it had mastered the ability to mount a warhead on a ballistic missile, ratcheting up a threat that its rivals and the United Nations have been powerless to contain. A U.S. special envoy met with Japanese officials on Sunday and said later 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. "The group of Obama's running around and talking about meaningless sanctions until today is highly laughable, when their 'strategic patience' policy is completely worn out and they are close to packing up to move out," state-run KCNA news agency cited a North Korean foreign ministry spokesman as saying in a statement later on Sunday. "As we've made clear, measures to strengthen the national nuclear power in quality and quantity will continue to protect our dignity and right to live from augmented threats of nuclear war from the United States," KCNA added. Earlier, the South's Yonhap news agency reported South Korea's military had a plan to use its missiles to "decimate" areas of Pyongyang if there were signs the North was about to launch a nuclear attack, quoting a source in the military. The South's Defence Ministry could not immediately confirm the report, but the military has vowed to take strong actions to retaliate in the event of an attack by the North. The North has yet to demonstrate that it had deployed nuclear-capable missiles, despite claims to have mastered the technology to miniaturise a nuclear warhead to mount it on ballistic missiles. NORTH KOREANS "DELIGHTED" 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. Obama said after speaking by telephone with South Korean President Park Geun-hye and with Japanese Prime Minister Shinzo Abe on Friday that they had agreed to work with the Security Council and other powers to vigorously enforce existing measures and to take "additional significant steps, including new sanctions". "We will be working very closely in the Security Council and beyond to come up with the strongest possible measure against North Korea's latest actions," said U.S. envoy Sung Kim on Sunday. "In addition to action in the Security Council, both the U.S. and Japan, together with the Republic of Korea, will be looking at unilateral measures, as well as bilateral measures, as well as possible trilateral cooperation," he said, referring to South Korea by its official name. South Korea's top nuclear envoy also spoke to his Chinese counterpart late on Saturday by telephone and emphasized the need for fresh countermeasures including a new U.N. security council resolution during their call, the South Korean foreign ministry said in a statement. 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. Another KCNA report on Sunday said North Koreans were "delighted" by the nuclear test. "The enemies can no longer deny the strategic position of our country as a nuclear weapons state," Jong Won Sop, a teacher at the University of National Economy, was quoted as saying.