South Korea has detected signs that the North is preparing for a fifth nuclear test, President Park Geun-hye said.
She did not elaborate on what the signs were, but ordered the military to be prepared, Yonhap news agency said.
It comes after reports in local media about increased activity at the North's Punggye-ri nuclear test site, where the previous nuclear tests took place.
A new test would be further defiance of tougher sanctions imposed by the UN last month.
"We are in a situation, in which we don't know whether North Korea could stage provocation as a move to overcome its isolation and to consolidate its internal unity," Ms Park said told her top aides, Yonhap reported.
In January, the North conducted its fourth nuclear test and a rocket launch a month later, raising tensions on the peninsula.
Observers have been saying another test could come before North Korea holds its Workers' Party congress in Pyongyang in May.
Experts believe the North does not possess the technology to mount a nuclear device on an intercontinental ballistic missile - although it has made progress in recent years on its nuclear weapons programme.