"Signs that it is preparing a fifth nuclear test have recently been detected," Park told a cabinet meeting.
Her remarks followed South Korean media reports in which unnamed government and intelligence officials spoke of a spike in activity at the North's Punggye-ri nuclear test site.
A fresh test would see Pyongyang doubling down in the face of tough sanctions adopted by the UN Security Council, and would throw down a gauntlet to the international community as it struggles to find new ways to curb the North's nuclear ambitions.
Numerous analysts have suggested the regime might carry out a fifth nuclear test as a display of defiance and strength just before the congress opens.
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"It is uncertain what kind of unexpected provocations it would stage," Park said at the cabinet meeting.
The South Korean Defence Ministry said it was alert to the likelihood of a fifth test.
"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," ministry spokesman Moon Sang-Gyun told reporters.
Tension has been high on the divided Korean peninsula since the January test and a rocket launch a month later that was widely seen as a disguised ballistic missile test.
The UN Security Council responded by imposing its strongest sanctions to date over the North's nuclear weapons programme.
Pyongyang has responded defiantly, staging a series of short- and mid-range missile tests and claiming a series of significant technical breakthroughs in its nuclear strike capability.
It claimed it had miniaturised a nuclear warhead to fit on a missile and successfully tested an engine designed for an inter-continental ballistic missile that could reach the mainland US.