The incident occurred Monday morning as an RC-135 reconnaissance aircraft flew in international airspace above the Baltic Sea.
It was intercepted by a Russian Su-27 Flanker fighter jet, which approached at a high speed.
The Russian fighter “came within several feet” of the RC-135, according to Capt. Joseph Alonso, a spokesman for U.S. European Command. A separate defense official said the fighter came within five feet of the RC-135 as it flew alongside it for several minutes.
The pilot of the American aircraft determined that the Russian aircraft’s actions were “unsafe” because of its “high rate of closure speed and poor control of the aircraft” said Alonso.
He added that the RC-135 was flying in international airspace with its transponder on, making it identifiable to other aircraft.
"We were flying in international airspace and did nothing to provoke this behavior," said Capt. Jeff Davis, a Pentagon spokesman, told reporters Tuesday.
"Such unsafe actions have the potential to cause serious harm and injury to all involved," he added. "Russia is certainly within its right to exercise within international airspace but we want them to respect international standards for safety to prevent accidents."
The Russian Defense Ministry countered with a different narrative of the intercept.
According to the Russian news agency Interfax the Russian Defense Ministry accused the RC-135 of performing provocative maneuvers toward the Russian fighter.
"During the escorting, the RC-135 crew made an attempt of closing with the Russian fighter, performing a provocative turnaround toward the Su-27," the agency quoted the Russian Defense Ministry as saying.
"The Russian pilot reacted to the maneuver of the RC-135 and then continued escorting the U.S. reconnaissance airplane until the latter changed the direction of its flight away from the Russian border," the ministry said.
The ministry added that after the 10-minute encounter, another RC-135 entered the same area and was also intercepted by a Russian Su-27 fighter.
After the incident, the RC-135 pilot determined the Russian's pilots actions were unsafe. Various factors go into determining whether an incident should be ruled unsafe including distance, speed, altitude, rate of closure and visibility.
Davis noted that the "vast majority" of intercepts of American aircraft by the Russian military are safe encounters. He labeled Monday's incident "the exception, not the norm."