Yonhap, citing South Korea's Joint Chiefs of Service, said North Korea had launched a ballistic missile towards the sea off the east coast of the Korean Peninsula (JCS).
The JSC did not immediately disclose details regarding the launch on Sunday.
The Japan Coast Guard stated that the object launched by North Korea may have been a ballistic missile.
North Korea is believed to have fired an intercontinental ballistic missile (ICBM) into the sea between the Korean Peninsula and Japan on Thursday, hours before South Korea's president went to Tokyo for a conference to discuss countermeasures against their nuclear-armed neighbour.
North Korean state media characterized the launch of the Hwasong 17 as a response to the current "frantic" joint exercises between South Korea and the United States.
After a year of record-setting North Korean nuclear tests and threats, Seoul and Washington have intensified their security cooperation.
Pyongyang considers all such drills as invasion rehearsals.
North Korea has declared itself an "irreversible" nuclear power. Its leader, Kim Jong Un, has recently called for an "exponential" rise in weapons development, especially nuclear tactical weapons.