North Korea's latest and most powerful nuclear test is stoking fears of the hermit state's significantly improved nuclear capability, even as it drew worldwide condemnation.
Its fifth test yesterday produced a 10-kilotonne blast in the country's north-east - almost twice as powerful as the one in January and not far from the 15-kilotonne atomic bomb dropped on Hiroshima in 1945.
South Korean President Park Geun Hye blasted the "fanatical recklessness" of the Kim Jong Un regime, saying it would lead North Korea "down the path of self-destruction". Japanese Prime Minister Shinzo Abe vowed more sanctions. China, the North's main ally, urged Pyongyang to stop taking actions that can worsen the situation.
US President Barack Obama warned of "serious consequences" and said he has agreed with US partners to take additional significant steps, including sanctions.
Singapore also expressed its concern, with the Ministry of Foreign Affairs saying the provocative act "seriously jeopardises the peace and stability of the region".
The United Nations Security Council, whose members include the US and China, was due to meet early this morning for urgent talks.
Calling yesterday's test a "success", Pyongyang's Nuclear Weapon Research Centre said it can now produce standardised nuclear warheads to be mounted on ballistic missiles, hinting that they are capable of mass production.
North Korea's enhanced nuclear capability could one day pose a serious threat to the world, experts said.
Over the years, sanctions and talks have failed to stop North Korea's nuclear ambitions. Instead of complying with calls from Washington and Seoul to give up nuclear weapons as a precondition to talks, Pyongyang has launched 22 ballistic missiles and conducted two nuclear tests this year. Analysts said North Korea has ramped up tests in a bid to gain recognition as a nuclear state and press for talks on its own terms.
"North Korea is single-minded in achieving the status and capability of a nuclear country," said Dr Bong Young Shik, a research fellow at Yonsei University's Institute for North Korean Studies. "Once it has nuclear weapons to threaten US forces deployed in South Korea, Guam and Japan, it's a trump card in future negotiations for diplomatic normalisation and economic aid."
The timing of the test, which coincided with the 68th anniversary of North Korea's founding, surprised many as it breaks the regime's three-year cycle of nuclear tests. Analysts did not expect another test so soon, as it takes time to analyse results and gather new raw materials.
This could mean Pyongyang has amassed enough nuclear ammunition and is using the test to "send a political message", said Dr Go Myong Hyun, a research fellow at The Asan Institute for Policy Studies.
"North Korea is basically saying ... unless you recognise me as a nuclear state and we sit down at the negotiation table as equals, we will keep conducting these provocations."
He added that North Korea could be feeling the pressure of UN economic sanctions imposed in March.
Analysts say frequent provocations may not work in a US election year. In fact, the latest test could provide further justification for the US to deploy its advanced anti-missile system in South Korea, a move opposed by China, they added.