US officials described the intercept by the SU27 jet on Wednesday as “dangerous and unprofessional”.
Russia’s defence ministry said the US plane had been approaching Russian land and the SU27 pilots had conformed to international rules.
Russia is carrying out military exercises in the Black Sea.
Pentagon spokesman Capt Jeff Davis said the US Navy p 8a Poseidon aircraft had been running regular operations in international airspace when the dangerous manoeuvre was made by the Russian fighter.
“These activities have the potential to unnecessarily escalate tensions, and could result in a miscalculation or accident,” he said.
An US defence official quoted by AFP news agency said the Russian airplane had flown within 30ft of the P8A before last to just 10ft.
On the other hand, 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 verification and to ascertain their wing numbers, the American aircraft changed course forcefully and flew away,” a statement said.
“The Russia aviators acted in strict accordance with international rules for flights.”
Relations between Russia and the West have been stressed since the Russian annexation of Crimea.
In April, the US complained that Russian jets had faced one of its reconnaissance planes over the Baltic Sea in an “unsafe and unprofessional fashion”.
For the reason that episode, Russia said that the US plane had turned off its transponder signal.