“Signs that it is preparing a fifth nuclear test have recently been detected,” Park said during a cabinet meeting.
She did not elaborate on what the signs where, but ordered the military to be prepared, Yonhap news agency said.
Citing South Korean government sources, media reports also said on Sunday that the North is preparing for the test to be carried out in early May.
South Korea’s military has said Pyongyang is technically ready for an additional nuclear test. The Defense Ministry also said the North can conduct its nuclear test at any time.
On Friday, a US monitoring website saw signs from satellite imagery that the North was looking to produce more plutonium for nuclear weapons.
“Given current activities, we believe that there is a possibility that the North may stage an underground nuclear test, and are monitoring the situation accordingly,” South Korean Defense Ministry spokesman Moon Sang Gyun told reporters.
In January, the North conducted its fourth nuclear test and a rocket launch a month later, raising tensions on the peninsula.