A Russian fighter jet carried out an "unsafe and unprofessional" intercept of a US spy plane over the Black Sea and came within 3.05 metres of the American aircraft, two US defence officials say.
The incident is likely to cause more tension between the United States and Russia, who are at odds over the Syrian civil war and Ukraine.
It lasted about 19 minutes and involved a Russian Sukhoi Su-27 fighter and a US Navy P-8 surveillance plane flying a regular patrol, said one official, speaking on condition of anonymity.
"They're up there for 12 hours and there are lots of interactions. But only one of the incidents was what the pilot determined was unsafe," said another official, who was not authorised to speak publicly.
Officials were talking with the pilot and reviewing the incident to determine whether it would be included in an annual meeting of US and Russian officials about more serious intercepts, the official added.
The Russian defence ministry said it had sent Su-27s on Wednesday to intercept a US aircraft approaching its border over the Black Sea because the American planes had turned off their transponders, which are needed for identification.
There have been a number of similar incidents involving Russia and the United States this year. In April, two Russian warplanes flew simulated attack passes near a US guided missile destroyer in the Baltic Sea.
The events are reminiscent of the Cold War, when a series of close calls led to a bilateral agreement aimed at avoiding dangerous interactions at sea that was signed in 1972.