The US has accused Russia of reckless behaviour after an American drone crashed into the Black Sea following an encounter with Russian fighter jets.
Before the collision, Russian jets are said to have dumped fuel into the path of the drone which the US says was in international airspace.
The damaged MQ-9 Reaper drone was brought down by the US after it became "unflyable", the Pentagon said.
Russia has denied its two Su-27 fighter jets made any contact.
The key question is whether the encounter was merely an attempt by Russia to disrupt the US drone and its work - or whether it was a deliberate attempt to bring it down.
US military officials said the incident happened at about 07:03 Central European Time (06:03 GMT) on Tuesday and the confrontation lasted around 30-40 minutes.
Several times before the collision, the fighter jets dumped fuel on the drone in a "reckless, environmentally unsound and unprofessional manner", it said in a statement.
