Ignoring repeated warnings from the international community to desist, North Korea is reported to have conducted an underground nuclear test early Monday, the South Korean news agency Yonhap said, citing the Korean Central News Agency (KCNA).
Radiation levels in the region are reported to be within norms.
"The situation is under control, and there is no danger to the local population," the spokeswoman said.
She added that experts were regularly monitoring and measuring radiation levels in the Primorye Territory, which borders on three Asian nations.
Pyongyang aroused serious concerns worldwide last Wednesday with claims that it would soon resume nuclear tests.
On Friday, the UN Security Council said that any nuclear test would be a serious threat to the peace and stability of the region.
North Korea conducted test launches of ballistic missiles in early July, including of a long-range Taepodong-2. Though the tests were a failure, many countries interpreted them as an attempt to force the international community, especially the United States, to make concessions in talks.
North Korea withdrew from the Nuclear Non-Proliferation Treaty in 2003, and in February 2005 announced it had acquired nuclear weapons. Some experts, however, questioned the claim.
Negotiations involving six nations - North Korea, South Korea, Russia, Japan, China and the United States - seek to persuade North Korea to give up its nuclear program. Talks began in 2003, but stalled last November.
Mediators proposed building a nuclear reactor for North Korea if it abandoned its nuclear program, but Pyongyang said it wants the reactor first and will then give up its nuclear research.
At the latest round of talks in September, 2005, North Korea agreed to abandon its nuclear program in exchange for aid and security guarantees, but later refused to return to the negotiating table until Washington lifted financial sanctions imposed on Pyongyang for its alleged involvement in counterfeiting and other illegal activities.
