Via the Korea Times:

North Korea appears to have conducted its second nuclear test Monday morning, Yonhap News Agency reported.

South Korea has detected an "artificial earthquake" in North Korea, raising the possibility that the communist state went ahead with its threat to conduct a nuclear test, Yonhap said, quoting a source.

"It was felt shortly before 10:00," the source said, declining to be named and adding that the magnitude was estimated at 4.5.

The Korean Central News Agency, the official news organ of the DPRK, has so far not reported the test. North Korea has been threatening to conduct another nuclear device test in reaction to tightened international sanctions after it fired a long-range rocket in April across the Sea of Japan. North Korea conducted its first test in October 2006.

According to Yonhap, the ROK news service, South Korean President Lee Myung-bak called an emergency meeting of his cabinet ministers over the test. No word yet from the Obama Administration.

Update [2009-5-24 23:53:14 by Charles Lemos]: According to the Associated Press, North Korea has confirmed that it conducted an underground nuclear test early on Monday morning, local time. The AP is quoting the country's official Korean Central News Agency as saying that Monday's test are "part of measures to bolster its nuclear deterrent for self-defense." But I cannot find any mention of any test on DPRK's news site.

A 4.7-magnitude earthquake was registered in northeastern North Korea at 9:54 a.m. (0054 GMT), the U.S. Geological Survey said. The quake, measured at a depth of 6 miles (10 kilometers) underground, occurred 40 miles (70 kilometers) northwest of the city of Kimchaek, the USGS said. The Japan Meteorological Agency also said it detected seismic activity Monday morning. "We are checking whether they were due to a nuclear test," agency official Gen Aoki said in Tokyo. In Seoul, the Korea Institue of Geoscience and Mineral Resources reported a 4.5-magnitude quake in Kilju in North Hamgyong Province. North Korea also carried out a nuclear test in October 2006 in Kilju, a test that drew sanctions from the United Nations and prompted five other nations to push negotiations on a nuclear disablement-for-aid pact with North Korea. In Washington, State Department spokesman Andy Laine said the U.S. government had no confirmation of a new nuclear test. "At this point we've seen the reports and we're trying to get more information, but we're not able to confirm at this time," Laine said.

The other response to watch for will be Beijing's. The test seems to have taken the world's intelligence agencies by surprise. More from the New York Times.

Via the New York Times, the full text of the English-language report on North Korea's KCNA news agency on Monday saying it had conducted a nuclear test:

"The Korean Central News Agency released the following report on Monday in connection with one more successful underground nuclear test in the DPRK."The Democratic People's Republic of Korea successfully conducted one more underground nuclear test on May 25 as part of the measures to bolster up its nuclear deterrent for self-defence in every way as requested by its scientists and technicians."The current nuclear test was safely conducted on a new higher level in terms of its explosive power and technology of its control and the results of the test helped satisfactorily settle the scientific and technological problems arising in further increasing the power of nuclear weapons and steadily developing nuclear technology."The successful nuclear test is greatly inspiring the army and people of the DPRK all out in the 150-day campaign, intensifying the drive for effecting a new revolutionary surge to open the gate to a thriving nation."The test will contribute to defending the sovereignty of the country and the nation and socialism and ensuring peace and security on the Korean Peninsula and the region around it with the might of (the military first policy) Songun."

Update [2009-5-25 3:7:18 by Charles Lemos]: The President issued a statement late Sunday night. It reads:

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. North Korea's attempts to develop nuclear weapons, as well as its ballistic missile program, constitute a threat to international peace and security. By acting in blatant defiance of the United Nations Security Council, North Korea is directly and recklessly challenging the international community. North Korea's behavior increases tensions and undermines stability in Northeast Asia. 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.