North Korea has fired multiple short-range cruise missiles into the sea.
South Korea's Joint Chiefs of Staff have said the airborne explosives landed between the Korea peninsula and Japan on Tuesday.
South Korea's military is monitoring for more developments,while its intelligence officials are, along with the US,conducting more analysis of launch, the JCS said.
It comes after Kim Jong-Un led North Korea had fired two ballistic missiles into the sea off the east coast of the Korean peninsula on March 20, South Korea's military claimed.
While earlier that month, North Korea had also apparently launched multiple missiles into the sea as part of firing drills.
This drew US and Chinese appeals for Pyongyang to return to talks on ending its nuclear and missile programmes.
The apparent short-range missiles were fired from North Pyongan province, South Korea's Joint Chiefs of Staff said.
The province is just above Pyongyang on the west coast of the Korean peninsula.
Japan's coast guard said on March 20 that North Korea appeared to have fired a missile, which landed outside Japan's exclusive economic zone waters.
State media KCNA said Kim Jong Un guided an artillery fire competition between combined units of the North Korean army.
North Korea had announced it would hold a session of Supreme People's Assembly, its legislature, in Pyongyang, this month.
Analysts said it would involve gathering almost 700 of the country's leaders in one spot as the coronavirus spreads worldwide.
