South Korea's spy agency, The National Intelligence Service, says North Korea has re-started a plutonium reactor at its main nuclear facility.
North Korea said it would re-start the reactor in April, but hasn't confirmed if it has done so since then.
Recent satellite images show signs the reactor may be operating.
A South Korean defense official cited the spy agency saying that North Korea also conducted missile engine tests at a northwestern site in August.
The agency did not say how it obtained the information and it did not confirm the official's remarks.