North Korea is ready to carry out another nuclear test at any time, South Korea's defence ministry said, three days after the North's fifth such test drew widespread condemnation.
Pyongyang set off its most powerful nuclear blast to date on Friday, 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.
"Assessment by South Korean and US intelligence is that the North is always ready for an additional nuclear test in the Punggye-ri area," South Korean defence ministry spokesman Moon Sang-gyun told a news briefing.
Punggye-ri, near the northeastern coast, is the site of all five of the North's nuclear explosions.
"North Korea has a tunnel where it can conduct an additional nuclear test," Moon said.
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South Korea's state news agency, Yonhap, reported earlier that North Korea had 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.
It did not elaborate on what activities had been detected at the Punggye-ri nuclear test site.
Al Jazeera's Harry Fawcett, reporting from Seoul, said it was already unprecedented that the North would carry out two tests in one year.
"There are some significant dates coming up: the 10th anniversary of the first test in October, the fifth anniversary of the death of Kim Jong-il in December," Fawcett said.
"And North Korea is undergoing a real acceleration of both its missile and nuclear programmes under the leadership of Kim Jong-un, and so this is the assessment as things stand of South Korea."
The US special envoy for the isolated state, Sung Kim, will travel to Seoul on Monday after discussing cooperation among neighbouring countries in Tokyo following the North's latest nuclear test.
Kim met Japanese officials on Sunday and said that the United States may launch unilateral sanctions against North Korea, echoing comments by US President Barack Obama on Friday after the test.
A push for further sanctions was "laughable", North Korea said on Sunday, vowing to continue to strengthen its nuclear power.
The US military delayed a planned B-1B bomber flight to the Korean peninsula, a show of strength and solidarity with ally Seoul, scheduled for Monday, Yonhap reported.
The delay of at least 24 hours was due to bad weather conditions in Guam, Yonhap said, citing an unidentified US Forces Korea official.
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South Korea's military put the force of Friday's blast at 10 kilotonnes, but a US expert said the highest estimates of seismic magnitude suggested a yield of 20 to 30 kilotonnes.
The test showed North Korea's nuclear capability was expanding fast and North Korean leader Kim Jong-un was unwilling to alter course, South Korea said on Saturday.
The UN Security Council denounced 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.
It was not clear if the Security Council can quickly come to a consensus on fresh sanctions, given the ambivalence by China and Russia on adopting another resolution.
Both countries did join sanctions in March after the North's January nuclear test.
Russian Foreign Minister Sergei Lavrov said on Saturday a "creative" response was needed.
The Global Times, run by the Chinese Communist Party, rejected the suggestion by the US that Beijing was responsible for the North's pursuit of nuclear arms.
It said the US was "the root cause" of the issue.
"China is not capable of persuading North Korea to give up nuclear development, because China's efforts are not supported by the others," it said in an editorial on Monday.
"Washington has been refusing to sign a peace treaty with Pyongyang."