The Pentagon says a Russian fighter plane flew within about 10ft (3m) of one of its reconnaissance aircraft operating over the Black Sea.
Russia’s defence ministry said the US plane had been approaching Russian territory and the SU-27 pilots had adhered to international rules.
Russia is currently carrying out military exercises in the Black Sea.
Pentagon spokesman Capt Jeff Davis said the US Navy P-8A Poseidon aircraft had been conducting routine operations in international airspace when the Russian fighter made the unsafe manoeuvre.
“These actions have the potential to unnecessarily escalate tensions, and could result in a miscalculation or accident,” he said.
A US defence official quoted by AFP news agency said the Russian plane had flown within 30ft of the P-8A before closing to just 10ft.
However, the Russian defence ministry said fighters intercepted the US plane because it was heading towards Russian territory with its transponder signal – which helps others identify it – switched off.
“After the Russian fighters got close to the spy planes for visual confirmation and to determine their wing numbers, the American aircraft changed course sharply and flew away,” a statement said.
“The Russia pilots acted in strict accordance with international rules for flights.”
Relations between Russia and the West have been strained since the Russian annexation of Crimea from Ukraine in March 2014.
In April, the US complained that Russian jets had confronted one of its reconnaissance planes over the Baltic Sea in an “unsafe and unprofessional manner”.
In that incident, Russia also said that the US plane had turned off its transponder signal.