The incident on Thursday occurred when a Russian jet "performed erratic and aggressive maneuvers" as it flew within 50 feet of the US aircraft's wing tip over the Baltic Sea, Danny Hernandez, a spokesman for European Command, said.
The Russian SU-27 began the barrel roll from the left side of the US RC-135 and went over the top of it to end on the right side of the aircraft, European Command said.
"The unsafe and unprofessional actions of a single pilot have the potential to unnecessarily escalated tensions between countries," said Hernandez, who added that the US is protesting the incident with the Russian government.
Pentagon spokeswoman Laura Seal said that the US aircraft had "at no time crossed into Russian territory."
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"There were no emergency situations," Konashenkov said.
At the same time, Russia said the SU-27 was dispatched to identify an "aerial target travelling toward the Russian border at high speed."
When the RC-135 established visual contact with the Russian jet, the American plane "changed its flight route away from the Russian border," Konashenkov said.
The encounter between the SU-27 and the RC-135 comes just days after the US Embassy in Moscow issued formal concerns with the Russian government over an incident in which Russian fighter jets flew very close to the USS Donald Cook in the Baltic Sea this week.