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.
Pentagon spokesman Brig Gen Pat Ryder told reporters the drone was “unflyable and uncontrollable so we brought it down”, adding that the collision also likely damaged the Russian aircraft.
He said the jets were able to land following the incident.
Reaper drones are surveillance aircraft with a 20m (66ft) wingspan.
Brig Gen Ryder said to his knowledge the Russians had not recovered the drone.
The United States’ National Security Council’s John Kirby told CNN: “Without getting into too much detail, what I can say is that we’ve taken steps to protect our equities with respect to that particular drone.”
Russia’s defence ministry said the drone crashed after a “sharp manoeuvre” and that it was flying with its transponders turned off. Transponders are communications devices that allow the aircraft to be tracked.
Prof Michael Clarke, visiting professor in war studies at King’s College, told the BBC the collision was “almost certainly an accident”.
The ex-director general of the defence and security think tank Royal United Services Institute said: “I don’t believe any pilot could deliberately do that [only colliding with another aircraft’s propeller] without seriously endangering their aircraft and own life.”
The incident highlights the increasing risk of direct confrontation between Russia and the US over the Ukraine war.
To protest against the move, the US summoned Russian ambassador Anatoly Antonov in Washington.
Following the meeting, Russian state media quoted Mr Antonov as saying that Moscow saw the drone incident as “a provocation” and that “the unacceptable activity of the US military in the close proximity to our borders is a cause for concern”.
Tensions have risen over the Black Sea ever since Russia’s annexation of Crimea in 2014.
Since Russia’s full-scale invasion of Ukraine the US and the UK have stepped up reconnaissance and surveillance flights, though always operating in international airspace.
According to the US, there has been a “pattern of dangerous actions by Russian pilots” interacting with allied aircraft in the region.
This means the downing of the drone could conceivably have been the result of a mistake by a Russian pilot who got too close as they “buzzed” the drone.
But if this was a deliberate attack on a US aircraft by a Russian warplane, then analysts say that would amount to a huge provocation and a substantial escalation.
In that case, the attack would be seen as an attempt by the Kremlin to test the response of the United States.
Western allies have worked hard to stop the war in Ukraine escalating into a direct confrontation with Russia. But this incident over the Black Sea is just that. The US will now have to evaluate its response.
As US military commanders warned in their statement, this was a dangerous act that “could lead to miscalculation and unintended escalation”.