"There are such signs," Unification Minister Ryoo Kihl-Jae told a parliamentary committee hearing when lawmakers asked him to confirm the reports.
The JoongAng Ilbo daily said South Korean intelligence had detected unusually busy personnel and vehicle movements at the North's Punggye-ri nuclear test site since last week.
"We are trying to figure out whether it is a genuine preparation for a nuclear test or just a ploy to heap more pressure on us and the US," it cited an unnamed South Korean government official as saying.
Intelligence reports also suggest Pyongyang has readied two mid-range missiles on mobile launchers on its east coast, and is aiming at a test firing before the April 15 birthday of late founding leader Kim Il-Sung.
A missile launch would be highly provocative, especially given the strong rebuke the North's sole ally China handed it on the weekend and a US concession to delay its own planned missile test.
However, Japan has ordered its armed forces to shoot down any North Korean missile headed towards its territory, a defence ministry spokesman in Tokyo said Monday.