The incident Thursday occurred when a Russian jet "performed erratic and aggressive maneuvers" as it flew within 50 feet of the U.S. aircraft's wing tip, Danny Hernandez, a spokesman for U.S. European Command, said in a response to a question from CNN.
The Russian Su-27 began the barrel roll from the left side of the U.S. RC-135 and went over the top of it to end on the right side of the aircraft, European Command said.
The RC-135 aircraft was "intercepted by a Russian SU-27 in an unsafe and unprofessional manner," Hernandez said, adding that the U.S. plane never entered Russian territory.
"The unsafe and unprofessional actions of a single pilot have the potential to unnecessarily escalate tensions between countries," said Hernandez, who added the United States is protesting with the Russian government.
Russia disputes reports
But Russia's Defense Ministry said that reports on Thursday's incident were "not consistent with reality" and that the Russian aircraft's maneuvers had been "performed strictly in accordance with the international regulations on the use of airspace."
Ministry spokesman Gen. Igor Konashenkov said the Su-27 had been dispatched after Russian air defense facilities spotted an unknown target over the Baltic Sea, approaching the Russian border at high speed.
It identified the jet as an American reconnaissance aircraft, and after visual contact, the U.S. plane "changed its course to the opposite direction, away from the Russian border," he said.
No incidents were recorded over the encounter, he said.
The encounter comes just days after the U.S. Embassy in Moscow issued formal concerns
with the Russian government over an incident last week in which Russian fighter jets flew close to the USS Donald Cook in the Baltic.
One of the Russian jets flew within 30 feet of the Cook's ship superstructure, according to a U.S. official.
Close encounters between Russian military aircraft and U.S. warships have become increasing common in recent months. In October, U.S. Navy jets intercepted two Russian Tu-142 aircraft
that were flying near the aircraft carrier USS Ronald Reagan in the Pacific Ocean.
In June, a Russian Su-24 jet flew within 500 meters
(1,640 feet) of a U.S. guided-missile destroyer that was sailing in the Black Sea near Crimea.
The Russian aerial maneuvers come amid rising tensions on NATO's eastern flank.
In February, the Department of Defense announced it was spending $3.4 billion for the European Reassurance Initiative in an effort to deter Russian aggression against NATO allies following Russia's 2014 intervention in Ukraine.
In recent weeks, the U.S. has deployed additional military assets throughout Europe as part of Operation Atlantic Resolve.
Earlier this month, the U.S. Air Force deployed F-15s to Iceland and the Netherlands
and F-22s to the United Kingdom
. And in February the United States announced it would send six F-15s to Finland
for a training exercise and pre-position tanks and artillery in Norway
. Both countries share a border with Russia.