Gird your loins, as they say:

North Korea staged a “successful” underground nuclear test, the communist state’s official media said.

The North “successfully conducted another underground nuclear test on May 25 as part of its measures aimed at strengthening its self-defence nuclear deterrent in every way,” the Korean Central News Agency said.

The test will “contribute to safeguarding our sovereignty and socialism and guaranteeing peace and safety on the Korean peninsula and the surrounding region,” it said. The four-paragraph story gave no details of the location. South Korea’s presidential office earlier said that North Korea had appeared to have conducted a nuclear weapons test. The government received a report on the possibility of a nuclear test having been conducted, presidential spokesman Lee Dong-Kwan told a briefing.

He said President Lee Myung-Bak called an emergency National Security Council meeting for Monday afternoon. “Both South Korea and US intelligence authorities are analysing and closely monitoring the situation,” the spokesman said.

(via AFP)

Earthquake sensors picked up the activity. Asian markets are down. Tensions are high:

The North’s official news agency, KCNA, said the country had conducted an “underground “ nuclear test.

The announcement came moments after the South Korean government geological sensors had detected an artificially triggered earth tremor emanating from Gilju, northeast North Korea, said Lee Dong-kwan, spokesman of the office of President Lee Myung-bak of South Korea.

The spokesman said “intelligence officials of South Korea and the United States are analysing the data and closely monitoring the situation.”

Word of the nuclear test sent a shudder through Asian financial markets, with Korea’s stock index plunging four percent within minutes.

North Korea conducted its first nuclear test on Oct., 9, 2006, and had given some advance notice of its intention to test a device. That initial test also was in the northeast.

North Korea recently threatened to conduct a second nuclear test, citing what it called Washington’s “hostilities” against the isolated Communist regime.

The test came against a backdrop of heightened tensions between North Korea and the United States, which keeps a heavy military deployment in South Korea and remains technically at war with the north 56 years after an armistice that halted the Korean War.Two American journalists are due to be tried June 4 in North Korea, charged with illegal entry into the North and “hostile acts.” That case in particular has aggravated tensions between Pyongyang and Washington, which were already strained after the North launched a long-range rocket on April 5.

Related reading: What is Obama cutting from defense? Why, missile defense, natch!

***

Japan to request an emergency U.N. Security Council meeting.

Emergency wrist-slap to follow.

***

Update 2:42am Eastern. This will have Kim Jong Il quaking in his boots!

Text of statement by President Barack Obama on nuclear and short-range missile tests conducted by North Korea.

Today, North Korea said that it has conducted a nuclear test in violation of international law. It appears to also have attempted a short range missile launch. These actions, while not a surprise given its statements and actions to date, are a matter of grave concern to all nations…

…Such provocations will only serve to deepen North Korea’s isolation. It will not find international acceptance unless it abandons its pursuit of weapons of mass destruction and their means of delivery.

The danger posed by North Korea’s threatening activities warrants action by the international community. We have been and will continue working with our allies and partners in the Six-Party Talks as well as other members of the U.N. Security Council in the days ahead.