MOSCOW, Aug. 8 -- At roughly 8:17 in the morning Greenwich time on Friday, a United States surveillance aircraft operating under the ARTEMIS II program completed its transit north from Romania and began tracing slow, methodical circles over Latvia and Estonia. The flight pattern continued for nearly three hours. The data comes from ADS-B Exchange, the flight-tracking service that aggregates transponder broadcasts from military and civilian aircraft across the continent.

The aircraft, a Bombardier Global 6000 configured for airborne electronic intelligence collection and operated by the United States military, had departed from Constanta Airport in Romania at approximately 5:42 GMT. It flew north in a wide arc through Hungarian, Slovak, a...