North Korea in December used a long-range rocket to send a satellite into orbit. Similar launches in 2006 and 2009 coincided with nuclear tests by Pyongyang.
A South Korean military official told the Yonhap news agency on condition of anonymity that satellite images of a test site in North Korea indicate it could detonate a nuclear device on short notice.
"North Korea has continuously conducted computer simulations for an additional atomic test with data acquired from the first and second nuclear tests," the senior official was quoted as saying.
The concerns follow a decision by the U.N. Security Council to censure Pyongyang for the December launch.
The resolution "demands" that North Korea avoid launches that may use ballistic missile technology and called on the government there to "abandon all nuclear weapons and existing nuclear program in a complete, verifiable and irreversible manner."
North Korea in early 2012 broke a pledge to halt nuclear testing by attempting an April rocket launch.
"This resolution demonstrates to North Korea that there are unanimous and significant consequences for its flagrant violation of its obligations under previous resolutions," U.S. Ambassador to the United Nations Susan Rice said in a statement.