The assessment came after North Korea fired multiple ballistic missiles toward the East Sea, with at least one traveling more than 900 kilometers (559 miles), according to Yonhap News Agency.
The Joint Chiefs of Staff (JCS) detected the launches around 9.20 am local time (0020GMT) from the Pyongyang area.
JCS spokesperson Jang Do-young said the launches could also be intended to demonstrate progress under North Korea's five-year defense development plan, Yonhap reported.
Jang declined to confirm whether the missiles were hypersonic or intermediate-range weapons, saying further technical analysis was required.
South Korea conducted its first test launch of a hypersonic glide vehicle Thursday, in an apparent demonstration of its advanced missile capabilities.
Japan also detected two suspected ballistic missiles, which reportedly fell outside its exclusive economic zone.
The latest launches come amid tensions following a Sept. 21 land mine explosion in the Demilitarized Zone that injured three South Korean soldiers, two of them seriously.
Seoul accused North Korean troops of deliberately planting the mines following a joint investigation with the UN Command.
Pyongyang denied responsibility, accused Seoul of fabricating the findings and warned of retaliation against any military counteraction.
North Korea has so far not issued a public statement on Monday's launches.