The defense white paper was released Friday detailing facilities where South Korea believes the North is working to produce weapons-grade uranium, Voice of America reported.
"According to satellite images jointly analyzed by South Korea and the United States, such [nuclear] activities are spotted around [highly enriched uranium] facilities," a senior intelligence official, whose name was not reported, said. "Although we have not yet concluded [about the possibility of a nuclear test], we are currently keeping track of the signs."
The paper said it is believed North Korea has about 40 kilograms of weapons-grade plutonium, enough for several weapons, Yonhap News Agency reported.
Georgetown University visiting professor Balbina Hwang said the new information is not a game changer.
"Countries have been operating under this assumption. And it was probably more than an assumption, in other words, that they had some sort of evidence," she said. "And it does nothing to change the U.S. attitude or probably the new government of South Korea's attitude towards North Korea."