Meanwhile Russia's military rejected the accusations, saying its jet was abiding by international laws and did not cause emergency situations.
The incident occurred on Thursday as a Russian Su-27 fighter "performed erratic and aggressive maneuvers", flying within 50 feet of a US RC-135 aircraft, US European Command spokesman Danny Hernandez said.
He added that the United States had protested to Moscow. "The unsafe and unprofessional actions of a single pilot have the potential to unnecessarily escalate tensions between countries," he said.
Russia dismissed the report as "running counter to reality", saying its air defences had to scramble a fighter jet after detecting a high-speed unidentified target over the Baltic Sea heading for its borders.
After making "visual contact" with the Russian Su-27, the American reconnaissance plane changed its course away from Russia's borders, defence ministry spokesman Igor Konashenkov said in a statement.
The flight of the Russian warplane was in "strict conformity with international laws... and there were no emergency situations," he said.
That incident occurred just two days after two Russian Su-24 bombers buzzed the Donald Cook, a US guided missile destroyer, in the Baltic Sea on Tuesday, simulating attack passes.