Via the New York Times:
North Korea said it had successfully conducted a nuclear test on Monday, raising the explosive power and level of control of its nuclear device to a new level, its state media said.
"We have successfully conducted another nuclear test on May 25 as part of the republic's measures to strengthen its nuclear deterrent," the North's official KCNA news agency said.
South Korea's Yonhap news agency also quoted a ruling party official as saying that a test was conducted.
US Geographical Survey recorded unusual seismic activity near the site of North Korea's previous test in 2006 shortly before the NK government's claims:
Unusual earthquake activity was reported in North Korea's nuclear test area on Monday morning, the United States Geological Survey reported. The agency said the earthquake, which happened at 9.55 a.m. local time, had a preliminary magnitude of 4.7.
The timing is interesting as it comes a day after the suicide of South Korean ex-prime minister Roh Moo-Hyun.