US European Command said two Russian Su-27 fighters intercepted the unmanned MQ-9 Reaper over international waters and one clipped its propeller.
"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," it said.
NATO diplomats in Brussels confirmed the incident, but said they did not expect it to immediately escalate into a further confrontation.