|North Korea's Yongbyon nuclear complex near Pyongyang.|
The North "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 Korean Central News Agency said.
"The current nuclear test was safely conducted on a new higher level in terms of its explosive power and technology of its control," it said.
The results resolved "scientific and technological problems arising in further increasing the power of nuclear weapons and steadily developing nuclear technology."
The brief report gave no details of the location of the test. South Korean officials said a tremor was detected around the northeastern town of Kilju, near where the first test was conducted in October 2006.
South Korean President Lee Myung-Bak called an emergency National Security Council meeting for Monday afternoon, while Japan's government announced a crisis task force had been set up.
The US government was not able to confirm "at this time" that a nuclear test had been carried out, a US State Department spokesman said.The North had threatened a second test in protest at the UN Security Council's decision to censure its April 5 long-range rocket launch.
It announced it was quitting six-nation nuclear disarmament talks and would restart its plutonium-making programme.
The communist state said Monday's test had greatly inspired the army and people.
It would "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 Songun (the North's army-first policy)."
The US Geological Survey said it detected what it called a 4.7-magnitude earthquake in North Korea on Monday.
The tremor struck at 9:54 am (0054 GMT), 375 kilometres (230 miles) northeast of Pyongyang at a depth of just 10 kilometres, it said.
"Both South Korean and US intelligence authorities are analysing and closely monitoring the situation," said a presidential spokesman in Seoul.
The test was staged while South Korea was in mourning for former President Roh Moo-Hyun, who leapt to his death on Saturday after being questioned in a corruption probe.
North Korea carried out what it called a rocket launch on April 5, but the United States, South Korea and Japan said it staged a disguised ballistic missile test.
After the UN Security Council condemned the launch and tightened sanctions, the North vowed to conduct a second nuclear test as well as ballistic missile tests unless the UN apologised.
The United States has been involved in negotiations with the two Koreas, China, Japan and Russia aimed at scrapping North Korea's nuclear programme in exchange for energy aid under a landmark six-party agreement signed in 2007.
The negotiations deadlocked over a dispute with North Korea over how to verify disarmament, before taking a sharp turn for the worse with the long-range rocket launch.