It was later revealed that the flying objects turned out to be a flock of birds. The DMZ, uninhabited for 66 years, is a haven for birds.
"Upon spotting unidentified object in the sky above DMZ, Air Force pilot was deployed and later confirmed that the trace was made by around 20 birds,” an officer from the Joint Chiefs of Staff told reporters in Seoul.
Trump became the first U.S. president to step into North Korea Sunday, reaching across the demarcation line to shake hands with Kim and then, at Kim's invitation, stepping across the border and into North Korea, a historic moment Trump called "a great honor."
Officials from both the U.S. and South Korea have insisted the meeting had not been prearranged.
ABC News' Hakyung Kate Lee and The Associated Press contributed to this report.