North Korea allegedly fired missiles with a multiple rocket launcher on Sunday.
South Korean forces observed "trajectories" that likely originated from North Korean artillery, the Seoul General Staff said.
The observations were made in the early evening local time.
How many trajectories were discovered remained unclear.
It was also unclear whether the projectiles were fired as part of ongoing summer military exercises by the North Korean People's Army.
The South Korean army has increased its surveillance and vigilance and remains fully operational, it said.
At the same time there is close cooperation with the USA.
The President's Office of National Security met and said it was "monitoring the situation very closely" in case there were any more kills.
North Korea has conducted numerous weapons tests since the beginning of the year, including one with an ICBM.
UN resolutions ban the country from testing nuclear-capable missiles.
The USA and South Korea also fear that North Korea could carry out a new nuclear weapons test at any time.
It would be the first such test since 2017. Pyongyang is subject to tough international sanctions because of its nuclear weapons program.
According to South Korea, North Korea's military last fired ammunition from a rocket launcher in June.