South Korea's military says it fired warning shots at an unidentified object flying from rival North Korea.
The military said in a statement Tuesday that it has bolstered its air surveillance after the incident but provided few other details.
South Korean news agency Yonhap reported, without citing a source, that South Korea fired about 90 machine gun rounds into the air and toward North Korea. It said South Korea was analysing whether a North Korean drone had crossed the border.
After a missile launch on Sunday, North Korea said the test met all technical requirements that could allow mass-production of the munition known as Pukguksong-2. Pyongyang said it had successfully tested what it called an intermediate-range ballistic missile.
US officials and experts, however, questioned the extent of its progress.
The test was North Korea's second in a week, and South Korea's new liberal government said it dashed hopes for peace on the peninsula.
China urged the DPRK not to violate UN Security Council resolutions with its nuclear and missile programmes.
The two Koreas, which signed an armistice not a peace treaty to end the 1950-1953 Korean War, face off across the world's most heavily armed border.
Tensions have increased in the region as Pyongyang continues to defy UN resolutions, and the US has ramped up its anti-DPRK rhetoric following the inauguration of Donald Trump as US president.