"Starting today, the military will maintain heightened vigilance against any provocations by the North," a Defense Ministry official was quoted by the Yonhap news agency as saying.
South Korea’s Foreign Ministry official told the outlet on condition of anonymity they believed the North was preparing to carry out a sixth nuclear test, after exploding a warhead in August.
Simultaneously, movement of vehicles and people has intensified at the Sohae military base in western North Korea, which has been used officially to launch satellites.
North Korea is celebrating the 71st anniversary of the ruling Workers Party on Monday. Pyongyang often uses important dates as a pretext for showing off its military power.