The US Air Force has released video said to show a Russian jet intercepting an American drone and dumping fuel on it over the Black Sea.
It said two Russian Su-27 jets flew close to the MQ-9 Reaper before one hit its propeller and forced remote operators to crash it into the ocean.
Tuesday’s incident has shown the increasing risk of direct confrontation between the superpowers as fighting continues in nearby Ukraine.
The jet’s manoeuvre was probably an attempt to “wash the fuel into the drone’s engine and therefore take it out”, said retired air vice-marshal Sean Bell.
“As he pulls the aircraft up, he hasn’t got the aerodynamics because he’s isn’t flying very fast… [the plane] gets very close to the body of the drone and almost certainly just clips the propeller,” he told Sky News.
Mr Bell called the pilot “incompetent” but said the collision was likely an accident because it also put the Russian plane in danger.
The MQ-9 Reaper – which has a 20-metre (66ft) wingspan and cost about $32m (£26m) – is often used for surveillance but can also be armed.
American officials have accused the Russian pilots of flying in a “reckless, environmentally unsound and unprofessional manner”.
Moscow has denied the jets behaved dangerously and said they didn’t come into contact with the drone, claiming it crashed due to “sharp manoeuvring”.
It said the Reaper was flying towards the Russian border with its transponders – wireless communication devices – turned off.
Russian defence minister Sergei Shoigu accused the US of ignoring flight restrictions imposed by Russia due to the Ukraine war.
America has insisted the drone was in international airspace and well clear of the conflict zone.
The seriousness of the incident has prompted the first calls since October between the countries’ defence secretaries and top commanders.
Russia has said it will try to recover the remains of the drone but no parts appear to have been recovered so far, a US official, speaking anonymously, told Reuters.
Washington said the aircraft came down in water about 1,200 to 1,500m deep and that measures had been taken to ensure no sensitive information could be gathered.
There are no American ships in the region so its own recovery attempts are unlikely, according to US officials.
A day after the incident, UK and German fighter jets were scrambled to intercept a Russian aircraft which failed to communicate with Estonian air traffic control.
The Ministry of Defence described it as a “routine” NATO mission.