The US and Russia have different accounts of what caused an American drone to crash into the Black Sea on Tuesday.
It is the first time the two powers have come into direct contact since the Ukraine war started last year.
American officials say two Russian fighter jets deliberately intercepted the ‘unmanned aerial vehicle’, while Moscow claims the US was flying too close to its territory.
Here Sky News looks at what we know so far.
The US regularly uses spy drones to gather intelligence around Ukraine, including the MQ-9 Reaper.
Reapers can be flown remotely up to 50,000ft. They are used primarily for surveillance but can be equipped with missiles for attack missions.
Defence analyst Professor Michael Clarke tells Sky News: “The Americans had a surveillance drone going round in circles over the Black Sea.
“It was almost certainly just surveying, picking up intelligence from on the ground. The Russians always try to put them off and buzz them with their aircraft.”
On this occasion, two Russian Su-27 fighter jets tried to intercept the drone by flying in front and dumping fuel on top of it in an effort to blind or damage it, US officials say.
This went on for 30 or 40 minutes on Tuesday morning before one of the Su-27s clipped the rear propeller of the Reaper somewhere between Crimea and Odesa.
Due to the damage, the Americans were forced to crash it into the water.
What has the US said?
The US Department of Defense has said the fighter jet’s contact with the drone forced it to crash land.
“Several times before the collision, the Su-27s dumped fuel on, and flew in front of the MQ-9 in a reckless, environmentally unsound and unprofessional manner,” US Air Forces Europe and Air Forces Africa commander James B. Hecker said.
The US says it was conducting routine operations in international airspace and that the Russian interception was “unsafe and unprofessional” and “demonstrates a lack of competence”.
Its European command also insists that while Reapers can carry weapons for attack missions, the one involved on Tuesday was only for intelligence.
What does Russia say?
The Kremlin says the American drone was flying too close to Crimea, which it annexed in 2014 and claims as its own territory.
Russia has also falsely declared the area a no-fly zone – describing the incident as a “provocation”.
Russian ambassador to the US, Anatoly Antonov, said: “The unacceptable actions of the United States military in the close proximity to our borders are cause for concern.
“If, for example, a Russian strike drone appeared near New York or San Francisco, how would the US Air Force and Navy react?”
Although the Su-27 carries a 30mm gun, Moscow says it “did not use airborne weapons” on this occasion.
It also claims its jet “did not come into contact” with the Reaper and the crash was a result of “sharp manoeuvring”.
Professor Clarke added: “In my mind, something clearly went wrong.
“Any pilot of a jet who could deliberately fly into a propeller of a drone that goes so slowly is a complete idiot.
“It’s much more likely that it was an accident and the pilot hit the propeller.”
What happens now?
While the Soviet-era jet managed to return back to base safely, the US drone has not been recovered.
According to Sky News security and defence editor Deborah Haynes: “The Americans will have done all they can to make sure that anything secret is not accessible, deleted, destroyed.”
Russian ambassador Mr Antonov was summoned to the US on Tuesday when he said Russia does not want any direct confrontation with Washington.
Professor Clarke says the “back and forth” between the two sides is likely to continue in the coming days.
“The US is gathering information all the time and you wouldn’t expect the Russians to like what’s going on and not try and disrupt it.
“That game has been going on for a couple of months now.
“But now instead of it just between a competition between US intelligence and Russian aircraft, it’s become an international incident.”