The plane, believed to be an RC-135W Rivet Joint, was detected in the skies above areas of Gyeonggi Province, including Namyangju, just north of Seoul, and the western port city of Incheon on Monday, Aircraft Spots posted on Twitter.
The operation took place on a day when North Korea called on the US to remove all "threats and hurdles" before discussing its denuclearization. Last week, Pyongyang offered to resume stalled talks later this month.
While making such a positive gesture, North Korea, on the other hand, ratcheted up tensions by test-firing short-range projectiles, and there has been concern over the possibility of additional launches.
So far this year, the communist country has conducted 10 major weapons tests.
Stepping up its surveillance of North Korea, the US has flown the reconnaissance plane over Seoul and its surrounding areas recently more often than before.
The aircraft used to occasionally fly over areas near Seoul around the time the North fired a string of ballistic missiles in 2017. But last year when the current peace process involving North Korea began, it was usually flown over the Yellow Sea.
The South Korean military has said it is monitoring the situation in case of the North's additional launches and maintaining a readiness posture. (Yonhap)