Under International Customary law, in circumstances such as this, an unidentified aircraft must be first warned of it encroaching into sovereign airspace. If this fails, then jets can be scrambled and the UFO can be forced to land. I note that the Turkish aircraft was shot in international airspace. I also note that the aircraft was an Unarmed RF-4E recon. aircraft.
U.S. confirms ISIS computer expert killed in air strike (Reuters)
from the article: Syria official says Damascus did not know downed warplane was Turkish