North Korea carried out an underground nuclear test on Monday, North Korea's Central News Agency (KCNA) reported.
The test was carried out at 0706 hrs IST in Hwaderi near Kilju city.
"Our science research section has safely and successfully conducted an underground nuclear test on October 9," it said.
It added that there was no leak or danger from the test.
South Korea's presidential Blue House said a tremor had been detected in North Korea on Monday.
It said South Korea's Institute of Geoscience and Mineral Resources had detected a tremor of a magnitude 3.58 to 3.7 at 07.00 am.
Officials in neighbouring Japan and China had no immediate comment on the reports.
North Korea had announced last week that it would test a nuclear device saying its hand was forced by what it called US threats of nuclear war and economic sanctions. But it said it would not be the first to use a nuclear weapon.
Following the announcement, South Korean President Roh Moo-hyun held an emergency meeting of top security officials at the Blue House.
Tokyo backs a hard line toward Pyongyang, while Seoul and Beijing - leery of instability on the peninsula - have previously cautioned against backing the North into a corner.
However, all three agree that Pyongyang should end its nearly year-long boycott of six-country talks on ending its nuclear weapons program.
Analysts say North Korea probably has enough fissile material to make six to eight nuclear bombs, but probably does not have the technology to devise one small enough to mount on a missile.