North Korea fired at least two unidentified missiles early on Thursday from an area near Wonsan, on North Korea's eastern coast.
The projectiles flew about 430 kilometres, South Korea's Joint Chiefs of Staff said.
"Our military is monitoring the situation in case of additional launches and maintaining a readiness posture," they told the Yonhap News Agency.
This would be the first launching of projectiles since US President Donald Trump and North Korean leader Kim Jong-un met at the demilitarised zone between the two Koreas at the end of June.
North Korea's last weapons test was in May, which included short-range missiles and smaller rockets.
That day Mr Kim oversaw the flight of a previously untested, relatively small and fast missile experts believe would be easier to hide, launch and manoeuvre in flight.
He and missile programme leaders also recently inspected a large, newly built submarine, the state news agency KCNA reported on Tuesday.
It could signal the continued development of a submarine-launched ballistic missile programme.
Talks between Mr Kim and Mr Trump on ridding the Korean Peninsula of nuclear weapons stalled after their second summit broke down in Vietnam in February.
