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The test ratchets up tensions between North Korea and the international community, and is the latest in a series of hard-line moves by Pyongyang.
In April it test-launched a long-range missile capable of reaching the United States. It is still holding two US journalists it accuses of spying. It has also kicked out UN inspectors and withdrawn from six-nation talks on its nuclear program.
It's not the first time North Korea has tested a nuclear device – it last did so in 2006.
But Reuters reported that Monday's underground test was a far more powerful explosion than that one. (Click here for a map of the suspected test location, from the Los Angeles Times, and a graphic on how such a test is conducted, from BBC.)
"[North Korea] successfully conducted one more underground nuclear test on May 25 as part of the measures to bolster up its nuclear deterrent for self-defense in every way," its [North Korea's] official KCNA news agency said.
The country's first test in October 2006 was considered to have been relatively weak, about 1 kilotonne, suggesting design problems. Russia's military said the latest test had a force of about 20 kilotonnes.
The international response has been swift, and negative. South Korea's president called an emergency meeting early Monday, after the Korea Meteorological Administration detected an "artificial earthquake" of 4.5 magnitude, according to the English-language Chosun Ilbo.
Japan, which called the test "absolutely impermissible," began working for an emergency UN Security Council meeting early Monday, and set up a special task force to deal with the crisis, according to the English-language news service of Kyodo News.
"North Korea's attempts to develop nuclear weapons, as well as its ballistic missile program, constitute a threat to international peace and security," [President Obama] said in a statement.
Citing South Korea's Yonhap News Agency, the Post reported that North Korea may have also fired a short-range missile into the sea, after the nuclear test.
A recent Chosun Ilbo article cited US and South Korean diplomats as saying that Pyongyang's recent saber-rattling may be related to internal power struggles and the succession question.
North Korean leader Kim Jong-Il is reportedly in ill health.
After his stroke last year, the question of the succession, for which no preparations had been made, suddenly came to the fore. As a result, it appears that the hardline military seized all the power it could and stoked international tensions to keep society under control.
"Since the appearance of health issues with Kim Jong-Il last year, the North Korean military became more influential," said Cheong Seong-chang, director of Inter-Korean Relations Studies Program at Sejong Institute near Seoul. ...
Cheong said he expected Pyongyang to soon indicate the nuclear test was in some part planned by Kim Jong-un, the North Korean leader's youngest son and frontrunner to take control of the secretive state.
"The outside world tends to underestimate Kim Jong-un at his young age," he [Cheong] said. "If Kim Jong-un played a decisive role in this nuclear test, it helps spread internally and externally a perception that he is a man of resolution."
In a Q&A posted on its website, the Guardian wrote that there's disagreement on how far North Korea has gotten with its nuclear program. But some now officially consider it one of Asia's nuclear powers.
The Associated Press reported that Asian markets "shrugged off" North Korea's test, with most posting gains Monday.
The region's markets have grown accustomed to such maneuvering by North Korea, said Linus Yip, a strategist at First Shanghai Securities Ltd. in Hong Kong.
"For the South Korean market, it's just an excuse for the market to make a correction because markets have shot up too much recently," Yip said. "But I don't see any great impact in other Asian markets."
According to a background report on North Korea's nuclear program by the Federation of American Scientists, tensions have been high since 2002, when the US told North Korea (and North Korea later admitted) that it was aware Pyongyang was enriching uranium for use in nuclear weapons.
That nullified the 1994 Agreed Framework, signed by the US and North Korea, under which North Korea was to freeze its nuclear program and allow inspectors, among other measures.