The US Air Force has accused the Russian military of carrying out an "unsafe" and "unprofessional" intercept above the Black Sea.
In a statement, the US Air Force said two Russian Su-27 flanker pilots flew within 100 feet of the American B-52 bomber multiple times last Friday.
The Russian aircraft's afterburners also caused turbulence and restricted the bomber's ability to manoeuvre, the statement added.
"Actions like these increase the potential for midair collisions, are unnecessary, and inconsistent with good airmanship and international flight rules," said General Jeff Harrigian, US Air Forces in Europe and Air Forces Africa commander.
"While the Russian aircraft were operating in international airspace, they jeopardised the safety of flight of the aircraft involved.
"We expect them to operate within international standards set to ensure safety and prevent accidents."
The US Air Force said the B-52 was conducting "routine" operations in international airspace at the time of the incident.