United States Air Force plane
The aircraft sent out a squawk code, which is used to provide effective communication between air traffic control and aircraft. 7700 is the most well known of these codes and it is used to indicate an emergency. The nature of the emergency is still unclear.
According to radarbox.com, the Boeing KC-135R Stratotanker took off from the RAF Mildenhall air station in Holywell Row, UK, at 05:36 am today (Wednesday).
The initial destination is not known.
The aircraft appears to have flown to Romania, almost reaching Bucharest before circling above Brasov.