The South Korean ministry of defence announced two unidentified projectiles were launched from Wonsan, North Korea, towards the east sea on Monday morning.
The statement from Seoul's joint chiefs of staff gave no further details about what had been sent into the air or how far they had flown.
"The military is monitoring for additional launches and maintaining readiness," officials said.
The announcement comes weeks after Pyongyang declared that its moratorium on nuclear and long-range missile tests was over.
In a December meeting, North Korean leader Kim Jong-un threatened a demonstration of a "new strategic weapon" soon. Mr Kim oversaw military drills on Friday, a rare public outing amid efforts to prevent an outbreak of the coronavirus in the isolated country.
North Korea carried out a series of weapons tests late last year, the last of them in November. Pyongyang often described them as multiple launch rocket systems, but others called them ballistic missiles. It also carried out static engine tests, most recently in December.
The regime has previously fired missiles capable of reaching the entire US mainland and has carried out six nuclear tests, the last of them 16 times more powerful than the Hiroshima blast, according to the highest estimates.
The country is under multiple sets of sanctions over its weapons programmes from the United Nations Security Council, the US, South Korea and other countries and organisations.
Heightened tensions in 2017 were followed by two years of nuclear diplomacy between Pyongyang and Washington, including three meetings between Mr Kim and US President Donald Trump, but little tangible progress was made.