The US has accused Russia of reckless behaviour after an American surveillance drone crashed into the Black Sea following an encounter with Russian fighter jets.
The US says the drone was on a routine mission in international airspace when two Russian jets tried to intercept it.
The incident highlights the increasing risk of direct confrontation between Russia and the US over the Ukraine war.
Russia denied that the two aircraft made any contact.
The Russian defence ministry said the drone crashed after a “sharp manoeuvre and that the MQ-9 Reaper drone was flying with its transponders turned off. Transponders are communications devices that allow the aircraft to be tracked.
Reaper drones are surveillance aircraft with a 20m (66ft) wingspan.
The incident happened at about 07:03 Central European Time (06:03 GMT) on Tuesday, according to the US military.
“Our MQ-9 aircraft was conducting routine operations in international airspace when it was intercepted and hit by a Russian aircraft, resulting in a crash and complete loss of the MQ-9,” the statement said.
Several times before the collision the Su-27 fighter jets dumped fuel on the drone in a “reckless, environmentally unsound and unprofessional manner”, it said.
The US summoned the Russian ambassador in Washington, Anatoly Antonov, to protest against the move.
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.
The key question is whether Tuesday’s encounter was an attempt by Russia to disrupt the US drone and its work, or whether it was a deliberate attempt to bring it down.
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”.